# Type operators

In standard Haskell, operator symbols may only be used at the value level. The only exception to this is the function arrow, `(->)`

, which is a built-in type operator. GHC expands the role of operators with the `TypeOperators`

extension, which makes it possible to use an operator as the name of a type.

The `TypeOperators`

extension has been available since GHC 6.8.1 and implies the `ExplicitNamespaces`

extension.GHC User Manual documentation for `TypeOperators`

.

## Defining type operators

By default, if you try to define a type whose name isn’t composed of letters, compilation fails and GHC suggests that you enable the `TypeOperators`

extension. This happens regardless of whether you place the name in infix or prefix position.

An example of failing with infix notation:

```
λ> data a & b = Tuple a b
error:
Illegal declaration of a type or class operator ‘&’ Use TypeOperators to declare operators in type and declarations
```

An example of failing with prefix notation:

```
λ> data (&) a b = Tuple a b
error:
Illegal declaration of a type or class operator ‘&’ Use TypeOperators to declare operators in type and declarations
```

With the extension enabled, either of these declarations is permitted.

```
λ> :set -XTypeOperators
λ> data a & b = Tuple a b
```

## Using type operators

You can *use* a type operator, in prefix notation, without any extensions enabled.

`λ> x = Tuple 2 'c' :: (&) Int Char`

However, you cannot use it in infix notation.

```
λ> x = Tuple 2 'c' :: Int & Char
error:
Illegal operator ‘&’ in type ‘Int & Char’ Use TypeOperators to allow operators in types
```

Using a type operator in infix notation requires enabling the `TypeOperators`

extension.

```
λ> :set -XTypeOperators
λ> x = Tuple 2 'c' :: Int & Char
```

## Example: Compose

In Functortown, we discuss the following example using the `Compose`

type:

```
{-# LANGUAGE DerivingVia #-}
import Data.Functor.Compose
newtype ReaderIO env a = ReaderIO (env -> IO a)
deriving (Functor, Applicative) via (Compose ((->) env) IO)
```

We emphasize that since type constructors are functions (at the type level), functor composition bears a great deal of similarity to function composition.Function composition: `(.)`

Function composition is often written using the infix operator `(.)`

. If we wanted our code to emphasize the similarity as well, we might want to define `(.)`

at the type level as an alias for the `Compose`

type constructor.

## Example: Generics

GHC generics`GHC.Generics`

defines types named `(:+:)`

and `(:*:)`

.