[SOLVED] Is there a GHC extension for enabling overloaded character literals?

Is there a GHC extension for enabling overloaded character literals?

I am aware that there is a GHC extension, OverloadedStrings, which allows string literals (delimited by ") to become polymorphic, similar to the built-in behavior for number literals.

My question is: is there a GHC extension that allows single character literals (delimited by ') to become polymorphic in an analogous way?

Solution

Not as of GHC 8.8, but you can use the QuasiQuotes extension to get pretty far. Here is an example of a quasiquote which only accepts an ascii character and converts it to its byte representation.

import Language.Haskell.TH
import Language.Haskell.TH.Quote
import Language.Haskell.TH.Syntax

import Data.Word (Word8)
import Data.Char (isAscii)

asciiByte :: QuasiQuoter
asciiByte = QuasiQuoter
  { quoteExp = \str -> case str of
                         [c] | isAscii c -> lift (fromIntegral (fromEnum c) :: Word8)
                         _ -> fail ("asciiByte: expects a single ascii character, got " ++ str) 
  , quotePat = \_ -> fail "asciiByte: only available for expressions"
  , quoteType = \_ -> fail "asciiByte: only available for expressions"
  , quoteDec = \_ -> fail "asciiByte: only available for expressions"
  }

Then, you can use it as:

ghci> [asciiByte|a|]
97
ghci> [asciiByte|é|]

<interactive>:75:12: error:
    • asciiByte: expects a single ascii character, got é
    • In the quasi-quotation: [asciiByte|é|]
ghci> [asciiByte|abc|]

<interactive>:76:12: error:
    • asciiByte: expects a single ascii character, got abc
    • In the quasi-quotation: [asciiByte|abc|]