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  • Glasgow Haskell Compiler
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  • #9173
Closed
Open
Created Jun 05, 2014 by Simon Peyton Jones@simonpjDeveloper

Improve type mismatch error messages

Generating better type-error messages is a hoary chestnut, but Herbert brought my attention to this thread on reddit, which has the following idea.

At the moment, from

module Foo where
  addThree = \x -> x + 3 :: Int
  y = addThree $ Just 5

we get this error:

Foo.hs:2:20
    Couldn't match expected type `Int' with actual type `Maybe a0'
    In the return type of a call of `Just'
    In the second argument of `($)', namely `Just 5'
    In the expression: addThree $ Just 5

Maybe we could generate this instead

Foo.hs:2:20
  inferred: "Just 5" has type "Maybe a0" 
  expected: second argument of "($)" must have type "Int"
  in the expression: addThree $ Just 5
Trac metadata
Trac field Value
Version 7.8.2
Type Bug
TypeOfFailure OtherFailure
Priority normal
Resolution Unresolved
Component Compiler
Test case
Differential revisions
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Blocking
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Operating system
Architecture
Edited Mar 10, 2019 by Alex D
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