GADTs should not allow polymorphism in return type

{-# LANGUAGE GADTs, RankNTypes, ImpredicativeTypes #-}

module Report (eval) where

data Taker a where
    Unreached :: Taker (forall s. s)
eval :: a -> Taker a -> (forall t. t)
eval x c = case c of
    Unreached -> x

I wanted the quantified types to match and the code to get past the type-checker, as it would if the forall type annotations were replaced with any ordinary type. However, ghc reported:

    Couldn't match expected type `t'
           against inferred type `forall s. s'
      `t' is a rigid type variable bound by
          the type signature for `eval' at ghc_report.hs:7:32
    In the expression: x
    In a case alternative: Unreached -> x
    In the expression: case c of { Unreached -> x }

I was unable to get around this problem by adding type annotations. However, I did get around it by using a similar type

data Z = Z (forall t.t)

instead of the bare forall type.

---

(yawl:~/programs/cat_type) scott% ghc -v -dcore-lint ghc_report.hs
Glasgow Haskell Compiler, Version 6.10.1, for Haskell 98, stage 2 booted by GHC version 6.10.1
Using package config file: /usr/lib/ghc-6.10.1/./package.conf
hiding package base-3.0.3.0 to avoid conflict with later version base-4.0.0.0
hiding package filepath-1.1.0.1 to avoid conflict with later version filepath-1.1.0.2
hiding package Cabal-1.6.0.1 to avoid conflict with later version Cabal-1.6.0.2
hiding package QuickCheck-1.2.0.0 to avoid conflict with later version QuickCheck-2.1.0.1
hiding package parsec-2.1.0.1 to avoid conflict with later version parsec-3.0.0
wired-in package ghc-prim mapped to ghc-prim-0.1.0.0
wired-in package integer mapped to integer-0.1.0.0
wired-in package base mapped to base-4.0.0.0
wired-in package rts mapped to rts-1.0
wired-in package haskell98 mapped to haskell98-1.0.1.0
wired-in package syb mapped to syb-0.1.0.0
wired-in package template-haskell mapped to template-haskell-2.3.0.0
wired-in package dph-seq[""] not found.
wired-in package dph-par[""] not found.
Hsc static flags: -static
Created temporary directory: /tmp/ghc26608_0
*** Checking old interface for main:Report:
*** Parser:
*** Renamer/typechecker:

ghc_report.hs:9:17:
    Couldn't match expected type `t'
           against inferred type `forall s. s'
      `t' is a rigid type variable bound by
          the type signature for `eval' at ghc_report.hs:7:32
    In the expression: x
    In a case alternative: Unreached -> x
    In the expression: case c of { Unreached -> x }
*** Deleting temp files:
Deleting: /tmp/ghc26608_0/ghc26608_0.s
Warning: deleting non-existent /tmp/ghc26608_0/ghc26608_0.s
*** Deleting temp dirs:
Deleting: /tmp/ghc26608_0

Trac metadata

Trac field	Value
Version	6.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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