隐藏 GHC 基础库以防止模式匹配脱糖到 GHC.Num.fromInteger 使用

Question

I have created a replacement Prelude for use in teaching beginning Haskell students, called FirstPrelude . One of the aims is to expunge type classes from the standard library so that error messages are more of the classic Hindley-Milner variety, rather than getting No instance errors. This is working out quite well. However, something I did not anticipate is that, when pattern matching, GHC is side-stepping my redefinition of fromInteger (defined as the identity, monomorphised to only work on Integer ) and so for example, with this function:

isZero 0 = True
isZero _ = False

If I ask GHCi for the type, I get:

isZero :: (GHC.Classes.Eq a, GHC.Num.Num a) => a -> Bool

But what I want is to get Integer -> Bool . Dumping the simplified core out of GHC I can see it is using:

(GHC.Num.fromInteger @Integer GHC.Num.$fNumInteger 0)))

I would have thought it would just use my fromInteger:: Integer -> Integer that is in scope, but alas no. Is there a way I can somehow prevent GHC.Num.fromInteger from being used? I guess perhaps this possible at the package level, but really I would love this at the module level for other pedagogical reasons.

Answer 1

This is not tied to pattern matching: In your Example.hs , even literals in expressions are polymorphic. To see this, write something like

simpler :: ()
simpler = 1

and observe the error message

[2 of 2] Compiling Main             ( Example.hs, interpreted )

Example.hs:7:11: error:
    • No instance for (GHC.Num.Num ()) arising from the literal ‘1’
    • In the expression: 1
      In an equation for ‘simpler’: simpler = 1
  |
7 | simpler = 1
  |           ^
Failed, one module loaded.

You may not have noticed because as soon as you use one of “your” operations, GHC specializes the type, and for top-level values may use defaulting.

But note

simpler _ = 1

for which it infers this type

ghci> :t simpler
simpler :: GHC.Num.Num p1 => p2 -> p1

If you enable {-# LANGUAGE RebindableSyntax #-} , it really uses “your” fromInteger and things work as you expect. You can do that in the .cabal file, maybe good enough for your educational needs.