#4940: Bad error message using poly pat bind with MonoPatBinds
---------------------------------+------------------------------------------
Reporter: batterseapower | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler (Type checker)
Version: 7.0.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: Incorrect warning at
compile-time
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This program:
{{{
foo :: a -> a
bar :: a -> a
(foo, bar) = (\x -> x, \y -> y)
main = print $ foo $ bar 1
}}}
Is trying to pattern-bind a polymorphic function. The error message is
dreadful:
{{{
/Users/mbolingbroke/Junk/PolyPatBinds.hs:5:15:
Couldn't match expected type `t -> t1'
with actual type `forall a. a -> a'
The lambda expression `\ x -> x' has one argument one argument,
but its type `forall a. a -> a' has none
In the expression: \ x -> x
In the expression: (\ x -> x, \ y -> y)
}}}
The issues are:
1. It repeats itself: "one argument one argument"
2. It is patently incorrect: the type forall a. a -> a DOES have one
argument in the standard nomenclature
3. It does not point the user towards the fix (-XNoMonoPatBinds)
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4940>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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