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  • Glasgow Haskell CompilerGlasgow Haskell Compiler
  • GHCGHC
  • Issues
  • #13610

Unhelpful error messages about lifted and unlifted types

I wrote this code:

{-# LANGUAGE MagicHash #-}
import GHC.Prim
import GHC.Types

main = do
    let primDouble = 0.42## :: Double#
    let double = 0.42 :: Double
    IO (\s -> mkWeakNoFinalizer# double () s)

and I get this error message:

WeakDouble.hs:8:15: error:
    • Couldn't match a lifted type with an unlifted type
      Expected type: (# State# RealWorld, Weak# () #)
        Actual type: (# State# RealWorld, Weak# () #)
    • In the expression: mkWeakNoFinalizer# double () s
      In the first argument of ‘IO’, namely
        ‘(\ s -> mkWeakNoFinalizer# double () s)’
      In a stmt of a 'do' block:
        IO (\ s -> mkWeakNoFinalizer# double () s)

with -fprint-explicit-kinds. (Without the flag, it looks the same, but tells me to use this flag…).

Trac metadata
Trac field Value
Version 8.1
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler (Type checker)
Test case
Differential revisions
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CC goldfire
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