The latest official version of the Haskell standard was published in 2010. Since then, GHC, the most popular Haskell compiler, has grown almost 100 extensions which modify the language, sometimes quite drastically. Many of them have proven popular, to the point where it’s rare to find a library on Hackage that doesn’t use any extensions.

This is a problem for students of Haskell since most teaching materials only cover the base standard, leaving learners to fend for themselves in the extension jungle. This guide aims to make the journey a little easier. To that end, I sort GHC’s extensions into four tracks:

- The basic track contains extensions that are generally simple and either commonly used or particularly useful for Haskell beginners. I also consider most of these extensions future-proof, in the sense that a future Haskell standard would probably include them without major changes.
- The advanced track contains extensions that are useful, but either more complex or less popular than the ones in the basic track. This includes a bunch of ‘special interest’ extensions that are only relevant if you want to do particular things with Haskell.
- The questionable track contains extensions that you probably shouldn’t use – either because they are actively unhelpful (badly designed or deprecated), or because they have failed to gain much adoption.
- The miscellaneous track contains extensions that wouldn’t fit in anywhere else.

Each track is subdivided into topics that are mostly independent, so you can pick and choose according to your interests. Which extension belongs into which track is of course a matter of opinion, so take mine with a grain of salt.

For each extension, I provide the following information:

- The GHC version which first introduced the extension. Note that some extensions have been changed considerably in later GHC versions, so there may be surprises lurking if you use extensions with older GHCs. When in doubt, consult the User’s Guide for your GHC version.
- An estimate of how stable the extension is. GHC doesn’t provide formal stability guarantees for extensions, so this is just my outside view of the situation. Possible classifications are:
*Stable*: the extension is well-established and I don’t expect its design to change much anymore.*Mostly stable*: The extension’s core design is stable, but details are somewhat likely to change.*Unstable*: it is at least possible that the extension will be changed significantly in future GHC versions.

- A link to the relevant section of the GHC User’s Guide. This is your primary reference, but it often contains a bunch of technical details that may obscure the ‘essence’ of an extension.
- Links to external tutorials where available. You should take a look at these before diving into the User’s Guide.
- A brief explanation of what the extension is all about. This is intended as a primer which allows you to determine if the extension is something worth knowing about. It’s not a full tutorial, and certainly not a reference.

Please let me know if I’ve made any mistakes. I don’t know many of these extensions too well myself, so there’s a good chance that some of the explanations are wrong. Also, if you have a suggestion for how to explain things more clearly (structure, choice of examples, etc.), I’d be grateful for that as well.

Of course, I’m not the first to cover the topic of GHC extensions. Some alternative takes:

- Stephen Diehl’s excellent, exhaustive and exhausting collection of knowledge, What I Wish I Knew When Learning Haskell, includes a list of extensions similar in spirit to this guide.
- Doug Beardsley provides a list of extensions sorted by how viable they are in a commercial setting.
- Alexis King’s Opinionated Guide to Haskell in 2018 contains a section with extensions that should be enabled by default.
- Alexander Altman has written a Guide to GHC Extensions for the School of Haskell, providing a mini-tutorial for each extension. For all extensions he covers, I provide a link in the relevant section of this guide.
- Oliver Charles (and guest authors) ran a blog post series on GHC extensions, called 24 Days of GHC Extensions, which I also frequently link to.

# Contents

- 1 Language Standards
- 2 Enabling GHC Extensions
- 3 Basic Track
- 4 Advanced Track
- 5 Questionable Track
- 6 Miscellaneous Track

# 1 Language Standards

There are essentially two versions of the Haskell standard: Haskell98 and Haskell2010. Haskell2010 is a conservative evolution of Haskell98 which just makes the language better, so there’s no reason to limit yourself to Haskell98. Consequently, this guide only covers extensions that aren’t in Haskell2010, and when I say “standard Haskell”, I mean Haskell2010.

Some day, we will hopefully see a new version of the standard that incorporates GHC’s more well-established extensions. Efforts in this direction are underway, but they’re moving so slowly that it’s hard to tell whether they’re moving at all. In the meantime, the pragmatic choice is to treat Haskell as an implementation-defined language and to use a conservative subset of GHC’s extensions.

# 2 Enabling GHC Extensions

It is good practice to specify exactly which extensions you are using. That way, if someone tries to compile your code with a different compiler (like GHCJS or Eta), the compiler can fail early if it doesn’t support all the extensions you rely on. There are two main ways to enable extensions:

You can enable extensions in a specific source file by adding a

`LANGUAGE`

pragma at the top of the file:If the file is part of a Cabal project, you should also add the extensions to your Cabal file’s

`other-extensions`

field(s). The`other-extensions`

field goes in a component block (`library`

,`executable`

,`test-suite`

or`benchmark`

) and should list all extensions used by the source files of that component.You can enable an extension globally in your project by adding it to a

`default-extensions`

field in your Cabal file:`library lib default-extensions: Extension1, Extension2 ...`

`default-extensions`

works like`other-extensions`

, but the listed extensions are enabled automatically for all source files belonging to the field’s component.

For some discussion on which style to prefer, see here, here and here.

# 3 Basic Track

## 3.1 Syntactic Niceties

### 3.1.1 `TupleSections`

Since 6.12 | Stable | User’s Guide | School of Haskell

Write `(x,,)`

instead of `\y z -> (x, y, z)`

.

### 3.1.2 `LambdaCase`

Since 7.6.1 | Stable | User’s Guide | School of Haskell

Instead of

write

### 3.1.3 `MultiWayIf`

Since 7.6.1 | Stable | User’s Guide | School of Haskell

Instead of

write

### 3.1.4 `TypeOperators`

Since 6.8.1 | Stable | User’s Guide

In Haskell2010, expressions like `(+)`

in a type are parsed as type variables, which is not very useful. This extension allows you to use operators as names for type constructors, type synonyms etc.:

Note that starting with GHC 8.6, using `*`

as a type operator may lead to trouble.

## 3.2 Overloaded Literals

### 3.2.1 `OverloadedStrings`

Since 6.8.1 | Stable | User’s Guide | School of Haskell | 24 Days

Overload string literals, similar to numeric literals. This means that `"a string"`

has type `IsString a => a`

, and you can define your own instances of `IsString`

. For example, a library for regular expressions might define an instance `IsString Regex`

that parses regular expressions, enabling you to write

### 3.2.2 `OverloadedLists`

Since 7.8.1 | Mostly stable | User’s Guide

Overload list literals, similar to numeric literals (and string literals with `OverloadedStrings`

). This means that `[1, 2]`

has type `(IsList l, Num (Item l)) => l`

. (`Item`

is an associated type of the `IsList`

class; see `TypeFamilies`

.) You can then define instances of `IsList`

for list-like types like vectors and sets and write

## 3.3 Patterns

### 3.3.1 `ViewPatterns`

Since 6.10.1 | Stable | User’s Guide | School of Haskell | 24 Days

A view pattern lets you apply a function as part of a pattern, then match against the result of that function. So instead of

```
toNonEmpty :: Set a -> Maybe (NonEmpty a)
toNonEmpty s = case toList s of
[] -> Nothing
(x:xs) -> x :| xs
```

write

```
toNonEmpty :: Set a -> Maybe (NonEmpty a)
toNonEmpty (toList -> [] ) = Nothing
toNonEmpty (toList -> (x:xs)) = x :| xs
```

### 3.3.2 `PatternSynonyms`

Since 7.8.1 | Mostly stable | User’s Guide | School of Haskell | 24 Days

Pattern synonyms allow you to define additional patterns for a type. For example, you can define a pattern that matches on the first two elements of a list:

```
pattern Head2 x y <- (x:y:_)
firstTwo :: [a] -> Maybe (a, a)
firstTwo (Head2 x y) = Just (x, y)
firstTwo _ = Nothing
```

Pattern synonyms are useful when you want to hide the representation of a datatype. For example, the `containers`

package defines a type `Seq`

representing finite lists. It is implemented as a special sort of tree, but the implementation is not exposed. Instead, the package defines pattern synonyms like `Empty`

and `:<|`

which allow you to match on a `Seq`

as if it were a list:

## 3.4 Type System

### 3.4.1 `ExplicitForAll`

Since 6.12.1 | Stable | User’s Guide | School of Haskell

The type signature `f :: a -> b -> b`

means that *for all* types `a`

and `b`

, we have a function from `a`

and `b`

to `b`

. For example, `f`

can be used as a function `Int -> Bool -> Bool`

, where `a := Int`

and `b := Bool`

. `ExplicitForAll`

allows us to make this “for all” explicit^{1} by writing

This is exactly the same type signature as above. Of course, that’s not very useful by itself, but the explicit `forall`

becomes relevant in combination with the next three extensions.

### 3.4.2 `TypeApplications`

Since 8.0.1 | Stable | User’s Guide | Kwang Yul Seo

A polymorphic function like

is really a function with *two* arguments: One type, `a`

, and one value of type `a`

. `TypeApplications`

allows you to give the type argument explicitly:

This is particularly useful when combined with GHCi’s `:t`

command, which you can use to view specialised type signatures:

```
> :t foldr
foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b
> :t foldr @[] @Int @Bool
foldr @[] @Int @Bool :: (Int -> Bool -> Bool) -> Bool -> [Int] -> Bool
```

### 3.4.3 `ScopedTypeVariables`

Since 6.8.1 | Stable | User’s Guide | 24 Days | School of Haskell

Consider the following slightly-contrived-for-demonstration-purposes implementation of the `minimum`

function:

```
minimum :: forall a. Ord a => [a] -> Maybe a
minimum [] = Nothing
minimum xs = Just $ foldl1' go xs
where
go :: a -> a -> a
go = min
```

GHC doesn’t accept this (even with `ExplicitForAll`

) because from its point of view, the `a`

in `minimum`

’s type signature and the `a`

in `go`

’s type signature are different variables that just happen to share the same name. Therefore, we don’t have a constraint `Ord a`

in `go`

, and so we can’t use `min`

(a method of `Ord`

).

With `ScopedTypeVariables`

, our definition of `minimum`

is accepted: Both `a`

s are now considered the same type variable, and the `Ord a`

constraint from `minimum`

‘trickles down’ to `go`

.

Caveat: This only works because we quantify over `a`

explicitly with `forall a`

. If we omit the `forall a`

, we get the same error as before.

### 3.4.4 `RankNTypes`

Since 6.8.1 | Stable | User’s Guide | School of Haskell | 24 Days | Gregor Riegler | Chris Done

The `ST`

monad can be used to safely run stateful computations. To run an `ST`

computation, you use the function `runST`

, which has type

This function has one type argument, `a`

, and one value argument of type `forall s. ST s a`

. `RankNTypes`

is required to allow this argument type, because it contains a `forall`

– without `RankNTypes`

, the only place where a `forall`

may appear is at the beginning of a type signature.

The meaning of this argument type is that the argument must be polymorphic in `s`

, meaning that the argument must have type `ST s a`

for *any* `s`

and *some* `a`

. Contrast this with the different type signature

This allows us to pass `m :: ST Int Bool`

as an argument to `runST`

, setting `a := Bool`

and `s := Int`

. With the correct signature of `runST`

, on the other hand, `m`

cannot be used as an argument because it would have to be of type `ST s Bool`

for *any* `s`

.

`RankNTypes`

is arguably an advanced extension, but it is required for `ST`

, which is in `base`

.

### 3.4.5 `LiberalTypeSynonyms`

Since 6.8.1 | Stable | User’s Guide | School of Haskell

Standard Haskell places a lot of restrictions on type synonyms; for example, you can’t use `forall`

in a type synonym. `LiberalTypeSynonyms`

lifts most of these restrictions, which can occasionally come in handy.

## 3.5 Records

### 3.5.1 `NamedFieldPuns`

Since 6.10.1 | Stable | User’s Guide | Wikibook

Suppose we have a record storing some configuration:

To extract only some data out of a `Configuration`

, we can use record pattern matching syntax:

```
localAddr :: Configuration -> String
localAddr Configuration { localHost = localHost, localPort = localPort }
= localHost ++ ":" ++ show localPort
```

Obviously, writing `field = field`

gets old quickly. `NamedFieldPuns`

reduces the noise:

```
localAddr :: Configuration -> String
localAddr Configuration { localHost , localPort }
= localHost ++ ":" ++ show localPort
```

### 3.5.2 `RecordWildCards`

Since 6.8.1 | Stable | User’s Guide | 24 Days | Kwang Yul Seo

This extension takes the principle behind `NamedFieldPuns`

one step further: We can now write (continuing the example):

```
localAddr :: Configuration -> String
localAddr Configuration { .. }
= localHost ++ ":" ++ show localPort
```

The `{ .. }`

is equivalent to writing one field pun for every field of `Configuration`

.

## 3.6 Classes

### 3.6.1 `FlexibleInstances`

Since 6.8.1 | Stable | User’s Guide

Haskell2010 doesn’t allow you to write instances like

because `Maybe`

must only be applied to type variables. `FlexibleInstances`

lifts this restriction. Additionally, it allows you to declare instances for type synonyms.

### 3.6.2 `FlexibleContexts`

Since 6.8.1 | Stable | User’s Guide

Haskell2010 places some restrictions on the superclass constraints that can appear in a class declaration. `FlexibleContexts`

lifts these restrictions.

### 3.6.3 `DeriveFunctor`

, `DeriveFoldable`

, `DeriveTraversable`

Since 7.10.1 | Stable | User’s Guide | 24 Days

This adds `Functor`

, `Foldable`

and `Traversable`

to the list of classes that GHC can derive for you automatically. So, if you ever need to define your own list type, you get almost every relevant function for free.

### 3.6.4 `GeneralizedNewtypeDeriving`

Since 6.8.1 | Stable | User’s Guide | 24 Days

In Haskell land, we like to introduce lots of `newtype`

s to prevent errors:

Yet that also forces us to write lots of boilerplate instances:

```
instance Num Dollars where
Dollars x + Dollars y = Dollars (x + y)
Dollars x * Dollars y = Dollars (x * y)
...
```

All we do here is wrap and unwrap the `Dollar`

constructor. With `GeneralizedNewtypeDeriving`

, GHC can write these instances for us:

and this work for almost every class, not just the built-in `Eq`

, `Ord`

etc.

There is an unfortunate interaction between `GeneralizedNewtypeDeriving`

and type families; see `RoleAnnotations`

. However, that doesn’t need to concern you most of the time.

### 3.6.5 `InstanceSigs`

Since 7.6.1 | Stable | User’s Guide

This extension allows you to give type signatures in instances, which can be useful documentation:

```
data Result a = Success a | Failure
instance Functor Result where
fmap :: (a -> b) -> Result a -> Result b
fmap f (Success x) = Success $ f x
fmap f Failure = Failure
```

Without `InstanceSigs`

, the signature for `fmap`

would be illegal.

### 3.6.6 `ConstrainedClassMethods`

Since 6.8.1 | Stable | User’s Guide

Haskell2010 doesn’t allow you to have additional constraints on class methods, so the following is disallowed (since `f`

, a class method, has an `Eq`

constraint):

`ConstrainedClassMethods`

lifts this (quoting the User’s Guide) “pretty stupid” restriction.

### 3.6.7 `MultiParamTypeClasses`

Since 6.8.1 | Stable | User’s Guide | 24 Days | Wikibook | Dennis Gosnell

In standard Haskell, classes can only apply to a single type (here `a`

):

`MultiParamTypeClasses`

lifts this restriction, allowing us to write classes that apply to multiple types. For example, the popular `mtl`

package defines a class

which denotes monads `m`

that have access to a value of type `r`

. (That’s not quite the whole story – `MonadReader`

needs an additional functional dependency.)

Type families provide a better alternative to `MultiParamTypeClasses`

in many cases, but the latter are older and still used by many popular packages.

### 3.6.8 `FunctionalDependencies`

Since 6.8.1 | Stable | User’s Guide | 24 Days | Wikibook | Dennis Gosnell

With `MultiParamTypeClasses`

, you’ll soon encounter situations where some of a class’s type parameters already determine another type parameter. For example, with `MonadReader`

, if `m`

is `Reader Int`

, then `r`

can only be `Int`

, and this applies generally: For every `m`

, there is at most one valid `r`

.

Functional dependencies let you express precisely this fact:

The `m -> r`

is a functional dependency which tells GHC that for any `m`

, there is at most one `r`

with instance `MonadReader m r`

(and GHC then won’t allow us to declare multiple such instances). This is useful because it allows GHC to infer `r`

from `m`

. Without the functional dependency, you would have to give a lot more type annotations because GHC would frequently fail to infer which `r`

you mean.

### 3.6.9 `DeriveGeneric`

Since 7.2.1 | Mostly stable | User’s Guide | 24 Days | Mark Karpov | Danny Gratzer | Haskell Wiki

`DeriveGeneric`

enables deriving of the `Generic`

class, which provides support for the popular `GHC.Generics`

flavour of generic programming. ‘Generic’ means that you can define functions which work for a variety of datatypes, without having to write separate code for each datatype.

The linked tutorials discuss how to write a library using generic programming, which is an advanced topic. However, to use these libraries (for example Aeson) you just need to put a `deriving Generic`

next to your datatypes, which is why this extension is in the basic track.

## 3.7 Typed Holes

### 3.7.1 Typed Holes and Type Wildcards

Since 7.8.1 | Mostly stable | User’s Guide | Chris Barrett | Haskell Wiki

Basic typed holes don’t require an extension and are enabled by default. They can occur in terms and in types.

In terms, you can write an underscore, called a hole, and GHC will tell you what type it expects you to fill in instead of the hole. For example, if you write

then GHC will throw an error saying that it expects something of type `a`

and that there is, conveniently, an `x :: a`

in scope.

In type signatures, you can write an underscore, called a type wildcard, and GHC will tell you what type it inferred. Continuing the example:

In this case, GHC will throw an error telling you that the `_`

must stand for `a`

. Of course, typed holes and type wildcards are much more interesting with more complex functions and types.

### 3.7.2 `NamedWildCards`

Since 7.10.1 | Mostly stable | User’s Guide | School of Haskell

This extension allows you to add an arbitrary name to a type wildcard, so you can write

GHC will use the name in the error message. You can also use the same named wildcard multiple times in a signature, in which case GHC assumes that all its occurrences refer to the same type:

Without this extension, `_a`

would be interpreted as a regular type variable.

### 3.7.3 `PartialTypeSignatures`

Since 7.10.1 | Mostly stable | User’s Guide | School of Haskell

When you use a type wildcard in a signature, you usually get an (informative) error. This extension turns these errors into warnings so you can use partial type signatures in your regular code. This lets you omit the boring parts of a complex type signature.

# 4 Advanced Track

## 4.1 Type System

### 4.1.1 `ExistentialQuantification`

Since 6.8.1 | Stable | User’s Guide | 24 Days | Wikibook | Arnaud Bailly | Jonathan Fischoff

An existentially quantified type allows you to ‘forget’ the type of a value, typically remembering only that the type belonged to some class. For example, we can define a type for any value that can be `show`

n:

```
data Showable = forall a. Show a => Showable a
intShowable :: Int -> Showable
intShowable n = Showable n
stringShowable :: String -> Showable
stringShowable s = Showable s
showShowable :: Showable -> String
showShowable (Showable x) = show x
```

As demonstrated by `intShowable`

and `stringShowable`

, a `Showable`

can contain a value of any type, as long as that type implements the `Show`

class. `showShowable`

demonstrates that we can use this fact to show a `Showable`

. Note that we can’t do anything else with a `Showable`

: the existential type ‘forgets’ everything about the value contained in it except that it can be shown.

Existential types look like an obvious solution to various problems, but it often turns out that these problems are better solved in other ways. Jonathan Fischoff’s article lays out some alternatives to existential quantification.

### 4.1.2 `GADTSyntax`

Since 7.2.1 | Stable | User’s Guide | School of Haskell

Constructors can be understood as special functions whose return type is the datatype to which they belong. For example, the `Maybe`

type has constructors

`GADTSyntax`

provides an alternative syntax for datatype declarations which reflects this understanding:

`GADTSyntax`

lets you define exactly those datatypes which you can define normally, so there’s not much reason to enable this extension, but we need it for `GADTs`

.

### 4.1.3 `GADTs`

Since 6.8.1 | Stable | User’s Guide | Wikibook | Haskell Wiki | Matt Parsons

When you write a regular datatype in `GADTSyntax`

, the return type of all constructors must be the datatype applied to its type variables. In our example from above, the return type of every constructor is (and must be) exactly `Maybe a`

:

GADTs lift this restriction, so different constructors can instantiate the datatype’s type variables differently:

```
data RestrictedMaybe a where
JustInt :: Int -> RestrictedMaybe Int
JustString :: String -> RestrictedMaybe String
Nothing :: RestrictedMaybe a
```

A `RestrictedMaybe`

can contain either an `Int`

or a `String`

, but no other types. This is not particularly useful, but the same principle can be applied to define a wide variety of interesting types.

### 4.1.4 `TypeFamilies`

Since 6.8.1 | Stable | User’s Guide | 24 Days | mchaver | Haskell Wiki | Matt Parsons | Kwang Yul Seo

Type families are the cornerstone of contemporary type-level programming in Haskell, so if you want to write programmes with fancy types (or use certain libraries), you should probably learn about them. The basics are not so hard, though there are a fair few corner cases. A short description couldn’t really do the concept justice, so I just recommend the above tutorials. Matt Parsons in particular gives a concise overview of type-level programming which also covers several other extensions.

### 4.1.5 `TypeFamilyDependencies`

Since 8.0.1 | Mostly stable | User’s Guide | Jan Stolarek | Paper (pdf)

Type family dependencies (aka injective type families) are the equivalent of functional dependencies for type families.

### 4.1.6 `AllowAmbiguousTypes`

Since 7.8.1 | Mostly stable | User’s Guide

GHC has an ambiguity check which raises an error if a function looks like it could never be called without leading to an ambiguous constraint. However, the check is incomplete, meaning that it sometimes rejects functions which can, in fact, be called. If you’re doing fancy type-level programming, you may therefore need to disable the check with `AllowAmbiguousTypes`

(but you probably don’t).

Despite the scary name, this extension does not compromise type safety. If a function call leads to an ambiguous constraint, this is still an error. The only difference is that the error is reported at the function call, not at the function definition.

## 4.2 Kinds

### 4.2.1 `KindSignatures`

Since 6.8.1 | Stable | User’s Guide

This extension allows you specify the kinds of type variables (wherever type variables may occur). This can be used for documentation, and it’s sometimes necessary when working with fancy kinds. An unnecessarily verbose example:

### 4.2.2 `ConstraintKinds`

Since 7.4.1 | Stable | User’s Guide | Kwang Yul Seo | Wolfgang Jeltsch

A constraint is anything that can appear to the left of the `=>`

arrow in a type signature (mostly type class constraints like `(Ord a, Eq b)`

). Constraints are type-like things which have a special kind, `Constraint`

. `ConstraintKinds`

makes this kind available, so you can define, for example, constraint synonyms:

```
type Serializable a = (Show a, Read a)
roundtrip :: Serializable a => a -> a
roundtrip = read . show
```

### 4.2.3 `DataKinds`

Since 7.4.1 | Mostly stable | User’s Guide | Matt Parsons | Kwang Yul Seo | Christian Marie

With `DataKinds`

enabled, when you define a data type like

you also get a kind `'Nat`

corresponding to the type `Nat`

, as well as types `'Zero`

and `'Succ`

corresponding to the constructors `Zero`

and `Succ`

. Hence

Together with type families, this allows you to compute, for example, natural numbers at the type level, much like you would at the value level.

### 4.2.4 `PolyKinds`

Since 7.4.1 | Mostly stable | User’s Guide

This extension enables kind-polymorphic types like the type-level identity function (which of course requires `TypeFamilies`

):

We can now apply `Id`

to types of different kinds:

### 4.2.5 `TypeInType`

Since 8.0.1 | Unstable | User’s Guide

`TypeInType`

merges the type and kind languages of GHC, meaning that types and kinds become the same thing. This implies that the type `*`

has kind `*`

; hence the extension’s name. It also means that everything we can do on the type level (type synonyms, type families, etc.), we can also do at the kind level.

## 4.3 Empty Types

### 4.3.1 `EmptyCase`

Since 7.8.1 | Stable | User’s Guide

Haskell2010 allows you to define empty data types, which have no constructors. The only value of such a type is `undefined`

. `EmptyCase`

allows you to match on such values with a `case`

statement that has zero alternatives, corresponding to the zero constructors:

### 4.3.2 `EmptyDataDeriving`

Since 8.4.1 | Mostly stable | User’s Guide

Haskell2010 does not allow you to derive the four standard type classes `Eq`

, `Ord`

, `Read`

and `Show`

for empty types. This is a simple oversight, which this extension fixes.

## 4.4 Classes

### 4.4.1 `StandaloneDeriving`

Since 6.8.1 | Stable | User’s Guide

`deriving`

clauses must usually be attached to a datatype declaration. This extension allows you to derive classes ‘after the fact’, even in a different module. Standalone `deriving`

clauses are also a little more liberal than attached `deriving`

clauses; for example, they support some GADTs.

### 4.4.2 `DefaultSignatures`

Since 7.2.1 | Stable | User’s Guide | Mark Karpov | Danny Gratzer | Haskell Wiki

When declaring a class, you can give default implementations for some or all of the class methods:

```
class Eq a where
(==) :: a -> a -> Bool
a == b = not (a /= b)
(/=) :: a -> a -> Bool
a /= b = not (a == b)
```

With `DefaultSignatures`

, you can give type signatures for default methods. These can be more specific than the implemented method’s signature, so you can provide a default implementation only if the type in question has instances of other classes. For example, one could define

```
class Eq a where
(==) :: a -> a -> Bool
default (==) :: Ord a => a -> a -> Bool
a == b = compare a b == EQ
instance Eq Int
instance Ord Int where
compare a b = ...
```

`DefaultSignatures`

is useful primarily for generic programming with `GHC.Generics`

.

### 4.4.3 `DeriveAnyClass`

Since 7.10.1 | Stable | User’s Guide

This extensions allows you to write a `deriving`

clause for any class, and GHC will simply generate an empty instance declaration (except for those classes which it knows how to derive). This makes sense if there are default implementations for all of the class’s methods, which happens frequently when using `GHC.Generics`

. For example, we may define a pretty-printing class which falls back to `Show`

(using `DefaultSignatures`

):

```
class Pretty a where
prettyPrint :: a -> String
default prettyPrint :: Show a => a -> String
prettyPrint = show
data Stuff = Stuff
deriving (Show, Pretty)
```

### 4.4.4 `DerivingStrategies`

Since 8.2.1 | Mostly stable | User’s Guide | Ryan Scott

With both `DeriveAnyClass`

and `GeneralizedNewtypeDeriving`

enabled, it is unclear how to process the following declaration:

GHC could either use the `DeriveAnyClass`

strategy and create an empty instance declaration, or it could use `GeneralizedNewtypeDeriving`

. `DerivingStrategies`

allows you to specify which strategy you want:

### 4.4.5 `UndecidableInstances`

Since 6.8.1 | Stable | User’s Guide | Dennis Gosnell

GHC places some restrictions on instance declarations to ensure that it can always resolve instances in finite time. These restrictions are incomplete, meaning that they disallow some instance declarations that are perfectly fine. `UndecidableInstances`

can therefore be used to disable GHC’s checks. You should not have to do this often.

### 4.4.6 `UndecidableSuperClasses`

Since 8.0.1 | Stable | User’s Guide | Edward Kmett (video)

GHC usually does not allow a class to be a superclass of itself, to ensure that typeclass resolution terminates. It can occasionally be useful to disable this check.

### 4.4.7 `RoleAnnotations`

Since 7.8.1 | Mostly stable | User’s Guide | Richard Eisenberg

GHC assigns every type variable (in datatypes, classes, etc.) a *role*. If a type variable has the wrong role, `GeneralizedNewtypeDeriving`

can be used to break type safety. GHC’s heuristics for role assignment mostly do the right thing, but it can be necessary to help them out by specifying roles explicitly. This extension allows you to do that.

Technically, you should think about roles whenever you write a type variable, but I’m not sure anyone actually does that.

## 4.5 Records

### 4.5.1 `DisambiguateRecordFields`

Since 6.8.1 | Stable | User’s Guide

Say you have two records with the same field name,

```
module M where
data UserPrefs = UserPrefs
{ user :: Int
, prefs :: String
}
-----------------------------------
module N where
data Profile = Profile
{ user :: Int
, profileData :: String
}
```

Haskell2010 will only let you use the `user`

record selector qualified, even where it’s entirely obvious which of the two record you mean. For example, the following is not accepted (with `M`

imported):

```
updateProfile :: (Int -> Int) -> Profile -> Profile
updateProfile f Profile { user = oldUser, profile = profile }
= Profile { user = f oldUser, profile = profile}
```

With `DisambiguateRecordFields`

, GHC accepts this definition (if and only if `UserPrefs`

and `Profile`

are defined in different modules). However, this only works because both occurrences of `user`

are under a `Profile`

constructor.

### 4.5.2 `DuplicateRecordFields`

Since 8.0.1 | Stable | User’s Guide

Extending `DisambiguateRecordFields`

, `DuplicateRecordFields`

allows multiple records *in the same module* to share a field name.

Moreover, GHC can now sometimes disambiguate uses of record fields based on type information. (`DisambiguateRecordFields`

, in contrast, only looks at whether record fields occur under a constructor.) For example, the following is accepted:

```
updateProfile :: (Int -> Int) -> Profile -> Profile
updateProfile f p = p { user = f $ user (p :: Profile) }
```

Exactly when GHC can disambiguate record fields is a little tricky – for instance, `updateProfile`

is not accepted without the type annotation on `p`

.

### 4.5.3 `OverloadedLabels`

Since 8.0.1 | Unstable | User’s Guide

With this extension, the new syntactic form `#foo`

is desugared to `fromLabel @foo`

, with `fromLabel`

a class method of

(`Symbol`

is a type-level string.) GHC by itself defines no instances of `IsLabel`

. As I understand it, this mechanism was intended to be used for properly overloaded record fields, but it seems like there is no satisfying solution to this problem yet.

## 4.6 Strictness

### 4.6.1 `BangPatterns`

Since 6.8.1 | Stable | User’s Guide | 24 Days | School of Haskell

With this extension, you can prefix a pattern with a bang (`!`

):

When something is matched against the bang pattern, it will be evaluated to weak head normal form before the body of `f`

is evaluated. For example, consider the following programme:

```
fibonacci :: Int -> Int
fibonacci 0 = 0
fibonacci 1 = 1
fibonacci n = fibonacci (n - 1) + fibonacci (n - 2)
main :: IO ()
main = print $ f (fibonacci 100)
```

Running `main`

will take a while because it has to (very inefficiently) compute `fibonacci 100`

before `f`

can return its result. Without the bang pattern, the programme terminates almost instantly because we don’t actually need the result of `fibonacci 100`

.

### 4.6.2 `Strict`

, `StrictData`

Since 8.0.1 | Mostly stable | User’s Guide | Johan Tibell

`StrictData`

makes all fields of datatypes strict by default. You can then use `~`

to make some fields lazy again. For example, with `StrictData`

active, the declaration

is equivalent to the usual

`Strict`

works like `StrictData`

, but in addition to fields of datatypes, it also makes most other things (patterns, `let`

/`where`

bindings, …) strict by default.

## 4.7 `Do`

Notation

### 4.7.1 `ApplicativeDo`

Since 8.0.1 | Mostly stable | User’s Guide | Paper (pdf)

A definition like

is equivalent to `f = g <$> mx <*> my`

, which uses only `Applicative`

combinators. Despite this, Haskell2010 will require a `Monad`

constraint – and use the monadic combinators – as soon as you use `do`

syntax. `ApplicativeDo`

changes this, allowing you to use `do`

notation with applicatives that aren’t also monads. Moreover, for some monads, the applicative combinators are more efficient than the monadic ones, in which case `ApplicativeDo`

may improve performance.

### 4.7.2 `RecursiveDo`

Since 6.8.1 | Stable | User’s Guide | 24 Days | Roman Cheplyaka | Will Fancher

Some monads (lists, `Maybe`

, `ST`

, `IO`

, …) support a notion of cyclic computation, in which a data structure is built by using parts of it that have already been built. This is captured in the `MonadFix`

class. `RecursiveDo`

adds some syntactic sugar so you can more easily write recursive monadic computations.

## 4.8 Literals

### 4.8.1 `NegativeLiterals`

Since 7.8.1 | Stable | User’s Guide

The literal `-1`

is usually desugared to `negate (fromInteger 1)`

. With `NegativeLiterals`

, it is instead desugared to `fromInteger (-1)`

. This should not make a difference most of the time.

### 4.8.2 `NumDecimals`

Since 7.8.1 | Stable | User’s Guide

Floating-point literals like `2.1e6`

usually have type `Floating a => a`

. With `NumDecimals`

, floating point literals which denote integers, like `2e6`

, have type `Num a => a`

instead, so you can use scientific notation for integers.

### 4.8.3 `BinaryLiterals`

Since 7.10.1 | Stable | User’s Guide

This extension allows you to write integer literals in binary: `0b10`

desugars to `fromInteger 2`

.

### 4.8.4 `HexFloatLiterals`

Since 8.4.1 | Mostly stable | User’s Guide

This extension allows you to write floating point literals in hexadecimal, which corresponds closely to the underlying binary representation.

## 4.9 Template Haskell

### 4.9.1 `TemplateHaskell`

, `TemplateHaskellQuotes`

Since 6.0/8.01 | Mostly stable | User’s Guide | 24 Days | Mark Karpov | Matt Parsons | Haskell Wiki

Template Haskell is a major extension that allows you to generate code at compile time. Basically, you write Haskell programmes that generate Haskell code (i.e. abstract syntax trees). GHC then runs your programmes at compile time and combines the generated code with your regular code. This is very powerful because you can generate whatever code you want, but it’s also a bit of a sledgehammer with a bunch of rough edges.

`TemplateHaskellQuotes`

enables a safer, but also considerably less useful subset of the `TemplateHaskell`

functionality.

### 4.9.2 `QuasiQuotes`

Since 6.10.1 | Stable | User’s Guide | Edsko de Vries

Quasi-quotes are an extension to Template Haskell which allow you to embed other languages into Haskell. For example:

`sql`

is, in effect, a function which parses an SQL query from a user-provided string at compile time, returning the Haskell type `Query`

.

### 4.9.3 `DeriveLift`

Since 7.2.1 | Stable | User’s Guide

Template Haskell makes use of a `Lift`

class to convert expressions into abstract syntax trees. This extension allows you to have your `Lift`

instances derived automatically.

## 4.10 Low-Level Hacking

### 4.10.1 `MagicHash`

Since 6.8.1 | Stable | User’s Guide

GHC likes to give its primitives names that end in a hash (`#`

). Haskell2010 disallows the hash in identifiers, so you have to enable `MagicHash`

to be able to refer to these primitives.

### 4.10.2 `UnboxedTuples`

Since 6.8.1 | Stable | User’s Guide | Michael Snoyman

An unboxed tuple, written `(# Int, Bool #)`

, allows you to return multiple values from a function without the overhead of a regular tuple (heap allocation, pointer dereferencing, etc.). When you return an unboxed tuple, its contents will be passed directly via registers or the stack.

### 4.10.3 `UnboxedSums`

Since 8.2.1 | Unstable | User’s Guide | Ömer Sinan Ağacan

An unboxed sum, written `(# Int | Bool #)`

, is like an anonymous sum type with multiple alternatives. Unlike regular sum types, GHC will try to represent unboxed sums as compactly as possible.

## 4.11 Safe Haskell

### 4.11.1 `Safe`

, `Trustworthy`

, `Unsafe`

Since 7.2.1/7.2.1/7.4.1 | Mostly stable | User’s Guide | Kristen Kozak (video with notes by Joe Nelson) | Edward Z. Yang | Paper (pdf)

Haskell has various features, like `unsafePerformIO`

, which can be (mis)used to circumvent the type system, module abstraction and other desirable properties. Safe Haskell allows you to control the use of these features to some degree, but I don’t think it ever really caught on.

## 4.12 Miscellaneous

### 4.12.1 `NoImplicitPrelude`

Since 6.8.1 | Stable | User’s Guide

The module `Prelude`

is usually imported implicitly in every module you write. `NoImplicitPrelude`

disables this. This is useful if you want to use one of many alternative preludes developed by the community, whose exported names frequently clash with those from `Prelude`

.

### 4.12.2 `RebindableSyntax`

Since 7.0.1 | Mostly stable | User’s Guide | 24 Days

Certain syntactic forms desugar to ordinary Haskell functions. For example, `do`

syntax is desugared into applications of the `>>=`

and `return`

combinators; literals `N`

are desugared into `fromInteger N`

; etc. Usually, this desugaring uses fixed functions, so the `fromInteger`

is really `Prelude.fromInteger`

. With `RebindableSyntax`

, the desugaring uses whatever functions are in scope, so you can define your own `fromInteger`

and GHC will use that.

### 4.12.3 `UnicodeSyntax`

Since 6.8.1 | Stable | User’s Guide | Haskell Wiki

This extension allows you to use Unicode symbols for certain keywords, like `∀`

instead of `forall`

. (Even without `UnicodeSyntax`

, you can use Unicode for identifiers.)

### 4.12.4 `NoMonomorphismRestriction`

Since 6.8.1 | Stable | User’s Guide | Neil Mitchell | Stack Overflow | Haskell Wiki

The monomorphism restriction is a highly technical feature of Haskell’s type system which sometimes makes GHC infer unexpected types for definitions without an explicit type signature. Since GHC 7.8.1, it is on by default in compiled modules, but off by default in GHCi. `NoMonomorphismRestriction`

allows you to switch it off even in compiled modules, but there are subtle disadvantages to this.

### 4.12.5 `PostfixOperators`

Since 7.10.1 | Stable | User’s Guide | School of Haskell

The operator section `(56 !)`

is usually equivalent to `\y -> (!) 56 y`

. With `PostfixOperators`

, it is instead treated as `(!) 56`

, so you can define and use postfix operators (though only with ugly parentheses):

### 4.12.6 `PackageImports`

Since 6.10.1 | Stable | User’s Guide

With this extension, an import can specify which package to import from:

This can be used if two packages you depend on define the same module, though that’s very rare in practice.

# 5 Questionable Track

## 5.1 Comprehensions

The extensions in this section aren’t particularly bad; they just have very few users as far as I can tell, and I don’t give them much of a chance to get in a future Haskell standard (except maybe `ParallelListComp`

).

### 5.1.1 `ParallelListComp`

Since 6.8.1 | Stable | User’s Guide | 24 Days

This extension extends list comprehension syntax so you can write

instead of

### 5.1.2 `TransformListComp`

Since 6.10.1 | Stable | User’s Guide | 24 Days

This extension adds SQL-inspired constructs like `group by`

and `using`

to the list comprehension syntax. I’ve never seen this used in the wild.

### 5.1.3 `MonadComprehensions`

Since 7.2.1 | Stable | User’s Guide | 24 Days

As it turns out, list comprehension syntax can be sensibly used with any monad, not just lists. `MonadComprehensions`

enables exactly that and includes the features from `ParallelListComp`

and `TransformListComp`

. I would argue, however, that the comprehension syntax loses much of its intuitive appeal when used with other monads.

## 5.2 Disabling Standard Features

### 5.2.1 `NoTraditionalRecordSyntax`

Since 7.4.1 | Stable | User’s Guide

This disables the usual record syntax, like `C { f = x }`

.

### 5.2.2 `NoPatternGuards`

Since 6.8.1 | Stable | User’s Guide

This disables pattern guards, which are a Haskell2010 feature similar to `ViewPatterns`

.

## 5.3 Miscellaneous

### 5.3.1 `Arrows`

Since 6.8.1 | Stable | User’s Guide | 24 Days

Arrows are a generalisation of monads. The `Arrows`

extension provides a notation for these constructs, similar to `do`

notation for monads. Outside of the Opaleye database library and certain ‘arrowised’ functional reactive programming libraries, arrows have not caught on in the community.

### 5.3.2 `StaticPointers`

Since 7.10.1 | Mostly stable | User’s Guide | 24 Days

Static Pointers allow you to get a reference to a closure (i.e. a computation, possibly with associated data) which can be serialised and deserialised. The intended use case is distributed programming, where computations can be turned into static pointers and then sent back and forth between nodes. However, as far as I know, the only user of this extension is Cloud Haskell.

### 5.3.3 `ImplicitParams`

Since 6.8.1 | Stable | User’s Guide

This extension allows you to have function parameters which are automatically propagated to calling functions. They have never caught on.

### 5.3.4 `ExtendedDefaultRules`

Since 6.8.1 | Mostly stable | User’s Guide | Kwang Yul Seo

The type of

is ambiguous: `sum [1..100]`

has type `(Num a, Enum a) => a`

and `show`

has type `Show a => a -> String`

– so which `a`

do we pick? `Int`

, `Integer`

and `Double`

would all be candidates. For convenience, Haskell has a defaulting mechanism which kicks in here: `a`

is defaulted to `Integer`

based on its constraints. (With `-Wtype-defaults`

, GHC warns you about this.) `ExtendedDefaultRules`

applies this principle to more classes than standard Haskell. This is intended for GHCi, where `ExtendedDefaultRules`

is enabled by default. In normal code, you usually want less defaulting, not more.

### 5.3.5 `DeriveDataTypeable`

Since 6.8.1 | Stable | User’s Guide | Chris Done

This extension allows you to derive instances of the `Data`

class automatically. This class supports “Scrap your Boilerplate”-style generic programming, which seems to have been mostly obsoleted by `GHC.Generics`

-style generic programming.

Recent GHCs derive instances of `Typeable`

without you even writing a `deriving`

clause, so you don’t need `DeriveDataTypeable`

for that anymore.

### 5.3.6 `NPlusKPatterns`

Since 6.12.1 | Stable | User’s Guide

n+k patterns are a misfeature of Haskell98 that has been removed for Haskell2010, so this extension is effectively deprecated.

### 5.3.7 `ImpredicativeTypes`

Since 6.10.1 | Unstable | User’s Guide

Impredicative types have been effectively unsupported for a while. The User’s Guide says that “GHC has extremely flaky support for impredicative polymorphism”, which is probably a euphemism.

## 5.4 Deprecated Extensions

### 5.4.1 `DatatypeContexts`

Since 7.0.1 | Stable | User’s Guide

Haskell2010 allows you to put a context on datatype declarations:

However, due to some weird design choices, this doesn’t work like you’d think and is pretty much useless.

### 5.4.2 `NullaryTypeClasses`

Since 7.8.1 | Stable | User’s Guide

A special case of `MultiParamTypeClasses`

.

### 5.4.3 `OverlappingInstances`

, `IncoherentInstances`

Since 6.8.1 | Mostly stable | User’s Guide

Consider the following situation:

The two instances overlap, meaning that if we look, say, for an instance `Foo [Int]`

, both instances match. This is disallowed by default. With `OverlappingInstances`

, it is allowed and the more specific instance (the second) will be chosen. With `IncoherentInstances`

, GHC doesn’t complain even if there is no single most specific instance; it will choose an arbitrary one.

`OverlappingInstances`

and `IncoherentInstances`

are deprecated since GHC 7.10.1 because the same functionality is now provided by the `OVERLAPPING`

, `OVERLAPPABLE`

, `OVERLAPS`

and `INCOHERENT`

pragmas (also explained in the linked section of the User’s Guide). These are preferable because they apply to a single instance, whereas the extensions disable the overlap checks for an entire module.

### 5.4.4 `Rank2Types`

Since 6.8.1 | Stable | User’s Guide

A more restricted form of `RankNTypes`

.

# 6 Miscellaneous Track

## 6.1 Extensions That Only Make Sense With Other Extensions

### 6.1.1 `ExplicitNamespaces`

Since 7.6.1 | Mostly stable | User’s Guide

With `TypeOperators`

, you can define the type `(++)`

. If you want to export this type while the function `(++)`

is also in scope, you need to be able to distinguish the two in the module export list. `ExplicitNamespaces`

allows you to do so, by writing

The same works when importing from `M`

. `ExplicitNamespaces`

also works with pattern synonyms, which can be prefixed with `pattern`

. Both `TypeOperators`

and `PatternSynonyms`

imply `ExplicitNamespaces`

, so you don’t need to enable it manually.

### 6.1.2 `MonoLocalBinds`

Since 6.12.1 | Stable | User’s Guide | Simon Peyton-Jones

`Let`

and `where`

bindings without an explicit type signature are usually generalised as much as possible. With `MonoLocalBinds`

, they are generalised a little less. This leads to more predictable type inference when using type families or GADTs, so these two extensions imply `MonoLocalBinds`

.

## 6.2 Temporary Extensions

### 6.2.1 `MonadFailDesugaring`

Since 8.0.1 | Unstable | User’s Guide

When a pattern match fails in a `do`

block, the `Monad`

class’s `fail`

method is called with an error message. For many monads, `fail`

is simply `error`

, which introduces implicit partiality, which is no good.

As part of the `MonadFail`

proposal, `do`

syntax will in future use a subclass of `Monad`

, `MonadFail`

, when a `do`

block contains a pattern that may fail. `MonadFailDesugaring`

enables this behaviour. The extension is temporary because it will eventually become standard.

Don’t let anyone tell you that Haskellers are bad at naming.↩