GitLab

When pattern matching, "let/where" and "case" have different behaviours.

Currently, this is accepted (a version using 'where' instead of 'let' type-checks too):

{-# LANGUAGE RankNTypes, ScopedTypeVariables #-}
module Main where

data Phantom x = Phantom Int deriving Show

foo :: forall y. (forall x. (Phantom x)) -> Phantom y
foo (Phantom x) = Phantom (x+1)
-- trying to map foo over a list, this is the only way I can get it to work
typeAnnotatedMap :: (forall x. [Phantom x])
                 -> [Phantom y]
typeAnnotatedMap intList = case intList of
                             [] -> []
                             _ -> let (phead:ptail) = intList
                                  in  foo phead : typeAnnotatedMap ptail

The following are not accepted:

typeAnnotatedMap1 :: (forall x. [Phantom x])
                 -> [Phantom y]
typeAnnotatedMap1 intList = case intList of
                             [] -> []
                             (phead:ptail) -> foo phead : typeAnnotatedMap1 ptail

typeAnnotatedMap2 :: (forall x. [Phantom x])
                 -> [Phantom y]
typeAnnotatedMap2 [] = []
typeAnnotatedMap2 (phead:ptail) = foo phead : typeAnnotatedMap2 ptail

The current type error is something like:

Couldn't match type ‘x0’ with ‘x’
      because type variable ‘x’ would escape its scope
    This (rigid, skolem) type variable is bound by
      a type expected by the context: [Phantom x]

More helpful would be something like:

Your pattern match bound the following types to a shared skolem variable:
      ptail :: [Phantom x0] (bound at rankNtest.hs:11:25)
      phead :: Phantom x0 (bound at rankNtest.hs:11:19)
You may have intended to use a "let" or "where" pattern-match instead.