How do you include a multiline number in a Haskell source file?

user_668074 0
Viewed 151

I would like to paste in a large number constant into my Haskell code, and for readability I would like to have it formatted over several lines instead of one line.

Is this possible?

syntax haskell
2 Answers

This is a possible approach. I'm not completely sure about it, though. There might be an easier way.

largeConstant :: Integer
largeConstant = read $
  "12345" ++
  "12345" ++
  "12345"

Alternatively, we could use multiline string literals, even though they are not very commonly used in Haskell.

largeConstant :: Integer
largeConstant = read
  "12345\
  \12345\
  \12345"

Enabling CPP is also an option, but seems a bit overkill.

largeConstant = 12345\
12345\
12345
user_32349590

You could make a quasiquoter. Might even want to upload it to hackage:

module X where

import Language.Haskell.TH
import Language.Haskell.TH.Quote
import Data.Char

iQQ :: QuasiQuoter
iQQ = QuasiQuoter {
  quoteExp  = return . LitE . IntegerL . read . filter isDigit,

  quotePat  = \_ -> fail "illegal integer QuasiQuote \
                         \(allowed as expression only, used as a pattern)",
  quoteType = \_ -> fail "illegal integer QuasiQuote \
                         \(allowed as expression only, used as a type)",
  quoteDec  = \_ -> fail "illegal integer QuasiQuote \
                         \(allowed as expression only, used as a declaration)"
  }

And the use:

{-# LANGUAGE QuasiQuotes #-}
import X

value = [iQQ|123
456|]

Resulting in:

Ok, two modules loaded.
*Main> value
123456

Or a larger number:

value = [iQQ|
44444444444444444444444444444444444444444444444444444444444444444444444444444444
45555555555555555555555555555555555555555555555555555555555555555555555555555555
66666666666666666666666666666666666666666666666666666666666666666666666666666666
|]

And in GHCi:

Ok, two modules loaded.
*Main> value
444444444444444444444444444444444444444444444444444444444444444444444444444444444555555555555555555555555555555555555555555555555555555555555555555555555555555566666666666666666666666666666666666666666666666666666666666666666666666666666666
user_2161640
Related