#2360: More information in occurs check message
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Reporter: ajd | Owner:
Type: feature request | Status: new
Priority: normal | Milestone: 6.10 branch
Component: Compiler | Version: 6.8.2
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Os: Unknown/Multiple
Architecture: Unknown/Multiple |
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Changes (by rwbarton):
* cc: rwbarton (added)
Comment:
Here are some more interesting cases. I'm specifically interested in
whether the error message pinpoints the location of the error; I call
those error messages good. My tests are under ghc-6.11.20090107.
Case 1: A good error message when there's no type signature
{{{
Prelude> let m x = x : x in m
<interactive>:1:14:
Occurs check: cannot construct the infinite type: a = [a]
Expected type: [a]
Inferred type: a
In the second argument of `(:)', namely `x'
In the expression: x : x
}}}
Case 2: A bad error message when a polymorphic type signature is given
{{{
Prelude> let m :: a -> a; m x = x : x in m
<interactive>:1:17:
Occurs check: cannot construct the infinite type: a = [a]
When generalising the type(s) for `m'
In the expression:
let
m :: a -> a
m x = x : x
in m
In the definition of `it':
it = let
m :: a -> a
m x = x : x
in m
}}}
Case 3: A good error when a monomorphic type signature is given
{{{
Prelude> let m :: Int -> Int; m x = x : x in m
<interactive>:1:31:
Couldn't match expected type `[Int]' against inferred type `Int'
In the second argument of `(:)', namely `x'
In the expression: x : x
In the definition of `m': m x = x : x
}}}
The difference between cases 2 and 3 surprises me because I could
mechanically turn case 2 into case 3 by substituting a fresh type
constructor for each universally quantified type variable.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2360#comment:5>
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