# Identifiers and operators

There are two kinds of names in Haskell: *Identifiers* and *operator symbols*.

## Identifiers

An *identifier* starts with a letter or underscore and consists of:

- Letters
- Numbers
`_`

(underscore)`'`

(single quote, sometimes pronounced as “prime”)

Examples of identifiers:

`x`

`x'`

`x'`

is pronounced as “x prime”`_x`

`π`

The Greek letter*pi*counts as a letter.`player2`

`player➆`

➆ is the unicode code point U+2786, entitled “dingbat circled sans-serif digit seven,” and it is considered a number.`Maybe`

`Just`

`Functor`

It is worth noting that the underscore in isolation is a reserved identifier, used as a wildcard in patterns. In some circumstances, a lone underscore may be interpreted as a hole.

## Operator symbols

An *operator symbol* consists entirely of “symbol or punctuation” characters. This includes the following ASCII characters:

`!`

`#`

`$`

`%`

`&`

`*`

`+`

`.`

`/`

`<`

`=`

`>`

`?`

`@`

`\`

`^`

`|`

`-`

`~`

`:`

as well as a great many Unicode characters. This means we can define operators using just about any symbol we want. There’s one exception to this: There are limitations on when we can use a colon (`:`

), which we discuss below. For example:

```
λ> 2 <♥> 3 6
```

## Prefix and infix

When a function is represented by an identifier, its application is typically written in *prefix* notation: the name of the function comes *before* the arguments. For example, a function named `f`

applied to arguments `x`

and `y`

looks like this:

` f x y`

For operator symbols, function application to two arguments is typically written in *infix* notation: the operator comes *between* the arguments. For example, a function named `+`

applied to arguments `x`

and `y`

looks like this:

`+ y x `

An identifier can be used in infix notation by enclosing it in backticks. So `f x y`

can be written equivalently as:

``f` y x `

Some examples of identifiers that are conventionally used in infix notation include `mod`

`Prelude.mod`

and `on`

: `Data.Function.on`

```
λ> 11 `mod` 3 2
```

```
λ> import Data.Function
λ> import Data.List
λ> sortBy (compare `on` fst) [(2, 'c'), (1, 'b'), (3, 'a')] [(1,'b'),(2,'c'),(3,'a')]
```

An operator can be used in prefix notation by enclosing it in parentheses. So `x + y`

can be written equivalently as:

`+) x y (`

## Capitalization matters

The *first* character of an identifier can only be a letter or an underscore, not a number or a single quote, with the underscore treated as a lowercase letter.

The following kinds of identifiers must begin with a upper case letter:

- Constructors
- Types
- Type constructors
- Type classes
- Module names

Everything else must begin with a lowercase letter or an underscore. This includes:

- Variables
- Type variables
- Functions and values other than data constructors
- Record fields

## Identifiers that end with a hash

Enabling the `MagicHash`

GHC extension slightly expands the set of names that are considered valid identifiers.

## Constructor operators start with a colon

If you’re going to use an operator symbol as the name of a data constructor, it must begin with a colon. Constructors are the *only* operators that are allowed to begin with a colon. The exception to this is type operators, which we discuss below.

For example, `NonEmpty`

has a single constructor named `(:|)`

.`Data.List.NonEmpty`

`data NonEmpty a = a :| [a]`

Note that this rule only applies to the *first* character in an operator name. A colon can always appear in another position. For example:

```
λ> 3 ^:^ 4 13
```

## Operators as type names

In standard Haskell, operator symbols are only for values; you can’t use an operator as the name of a type. If you enable the `TypeOperators`

GHC extension, then you can.

The rule about starting with a colon doesn’t apply at the type level. Type operators may or may not begin with a colon.

The only exception is `->`

, which is a built-in type operator that you can use without the `TypeOperators`

extension.