Overloaded Literals

Strings

The type of the literal

Without any extensions, the type of a string literal – i.e., something between double quotes – is just a string, aka list of characters:

Prelude> :t "foo"
"foo" :: [Char]

However, when the OverloadedStrings extension is enabled, string literals become polymorphic, similar to number literals:

Prelude> :set -XOverloadedStrings
Prelude> :t "foo"
"foo" :: Data.String.IsString t => t

This allows us to define values of string-like types without the need for any explicit conversions. In essence, the OverloadedStrings extension just wraps every string literal in the generic fromString conversion function, so if the context demands e.g. the more efficient Text instead of String, you don’t need to worry about that yourself.

Using string literals

{-# LANGUAGE OverloadedStrings #-}

import Data.Text (Text, pack)
import Data.ByteString (ByteString, pack)


withString :: String
withString = "Hello String"

-- The following two examples are only allowed with OverloadedStrings

withText :: Text
withText = "Hello Text"      -- instead of: withText = Data.Text.pack "Hello Text"

withBS :: ByteString
withBS = "Hello ByteString"  -- instead of: withBS = Data.ByteString.pack "Hello ByteString"

Notice how we were able to construct values of Text and ByteString in the same way we construct ordinary String (or [Char]) Values, rather than using each types pack function to encode the string explicitly.

For more information on the OverloadedStrings language extension, see the extension documentation.

Floating Numeral

The type of the literal

Prelude> :t 1.0
1.0 :: Fractional a => a

Choosing a concrete type with annotations

You can specify the type with a type annotation. The only requirement is that the type must have a Fractional instance.

Prelude> 1.0 :: Double
1.0
it :: Double
Prelude> 1.0 :: Data.Ratio.Ratio Int
1 % 1
it :: GHC.Real.Ratio Int

if not the compiler will complain

Prelude> 1.0 :: Int
<interactive>:
    No instance for (Fractional Int) arising from the literal `1.0'
    In the expression: 1.0 :: Int
    In an equation for `it': it = 1.0 :: Int

Integer Numeral

The type of the literal

Prelude> :t 1
1 :: Num a => a

choosing a concrete type with annotations

You can specify the type as long as the target type is Num with an annotation:

Prelude> 1 :: Int
1
it :: Int
Prelude> 1 :: Double
1.0
it :: Double
Prelude> 1 :: Word
1
it :: Word

if not the compiler will complain

Prelude> 1 :: String

<interactive>:
    No instance for (Num String) arising from the literal `1'
    In the expression: 1 :: String
    In an equation for `it': it = 1 :: String

List Literals

GHC’s OverloadedLists extension allows you to construct list-like data structures with the list literal syntax.

This allows you to Data.Map like this:

> :set -XOverloadedLists
> import qualified Data.Map as M
> M.lookup "foo" [("foo", 1), ("bar", 2)]
Just 1

Instead of this (note the use of the extra M.fromList):

> import Data.Map as M
> M.lookup "foo" (M.fromList [("foo", 1), ("bar", 2)])
Just 1

Remarks

Integer Literals

is a numeral without a decimal point

for example 0, 1, 42, …

is implicitly applied to fromInteger which is part of the Num type class so it indeed has type Num a => a - that is it can have any type that is an instance of Num

Fractional Literals

is a numeral with a decimal point

for example 0.0, -0.1111, …

is implicitly applied to fromRational which is part of the Fractional type class so it indeed has type a => a - that is it can have any type that is an instance of Fractional

String Literals

If you add the language extension OverloadedStrings to GHC you can have the same for String-literals which then are applied to fromString from the Data.String.IsString type class

This is often used to replace String with Text or ByteString.

List Literals

Lists can defined with the [1, 2, 3] literal syntax. In GHC 7.8 and beyond, this can also be used to define other list-like structures with the OverloadedLists extension.

By default, the type of [] is:

> :t []
[] :: [t]

With OverloadedLists, this becomes:

[] :: GHC.Exts.IsList l => l