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haskelltypesliterals
Owen
Owen

Reputation: 39356

In Haskell can I make numeric literals not polymorphic by default?

This is probably not possible, since I already checked the list of all GHC extensions and this is not in there, but I thought I'd ask just in case.

Is there any way to make it so that 2 has type Int (or Integer) rather than the usual Num a => a?

(The reason I'd like this behavior is that it makes error messages clearer and type inference more likely to be possible (esp with type classes). I could always write (2::Int) everywhere but I'd rather the "more safe" behavior be the less explicit one)

Upvotes: 11

Views: 1669

Answers (2)

Chris Kuklewicz
Chris Kuklewicz

Reputation: 8153

I think I should add "default()" to these answers, though I think gatoatigrado mentioned it in passing. The Haskell 98 standard has a section 4.3.4 which eventually describes how to alter some defaulting of Num a => a values. The implicit defaulting order is given by

 default (Integer, Double)

and can be changed, e.g. by putting

 default (Int)

or

 default ()

in the source file.

Upvotes: 4

C. A. McCann
C. A. McCann

Reputation: 77374

There is a (slightly abusive and inconvenient) way to do this using GHC extensions.

{-# LANGUAGE RebindableSyntax #-}

import qualified Prelude as P
import Prelude hiding (Num(..))

fromInteger :: Integer -> Integer
fromInteger = id

In GHCi:

> :set -XRebindableSyntax
> :t 2
2 :: Integer

With the RebindindableSyntax extension enabled, GHC will use whatever fromInteger is in scope to handle numeric literals. The only constraint is that it must take an argument of type Integer (actually, even this isn't required, but if it doesn't you'll get a type error from numeric literals).

Note that, because the standard fromInteger is part of the Num class, you may need to hack some things around to get things working properly.

Upvotes: 14

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