#2588: Missed suggestion with context on foralls
--------------------------+-------------------------------------------------
Reporter: NeilMitchell | Owner:
Type: bug | Status: new
Priority: normal | Milestone: 6.12 branch
Component: Compiler | Version: 6.9
Severity: normal | Resolution:
Keywords: | Difficulty: Unknown
Testcase: | Architecture: Unknown
Os: Unknown |
--------------------------+-------------------------------------------------
Changes (by simonpj):
* difficulty: => Unknown
* milestone: => 6.12 branch
Old description:
> Given the program:
>
> {{{
> module Test where
> test :: forall a . a -> a
> test = id
> }}}
>
> I get the nice and helpful error message:
>
> {{{
> Test.hs:3:17:
> Illegal operator `.' in type `forall a . (a -> a)'
> Perhaps you intended to use -XRankNTypes or similar flag
> to enable explicit-forall syntax: forall <tvs>. <type>
> }}}
>
> [I personally usually want ScopedTypeVariables rather than RankNTypes,
> but its still a very useful hint at where to look]
>
> However, trying the program:
>
> {{{
> module Test where
> test :: forall a . Eq a => a -> a
> test = id
> }}}
>
> Gives:
>
> {{{
> Test.hs:3:24: parse error on input `=>'
> }}}
>
> I think it would be helpful to give the same suggestion as above.
>
> -- Neil
New description:
Given the program:
{{{
module Test where
test :: forall a . a -> a
test = id
}}}
I get the nice and helpful error message:
{{{
Test.hs:3:17:
Illegal operator `.' in type `forall a . (a -> a)'
Perhaps you intended to use -XRankNTypes or similar flag
to enable explicit-forall syntax: forall <tvs>. <type>
}}}
[I personally usually want `ScopedTypeVariables` rather than `RankNTypes`,
but its still a very useful hint at where to look]
However, trying the program:
{{{
module Test where
test :: forall a . Eq a => a -> a
test = id
}}}
Gives:
{{{
Test.hs:3:24: parse error on input `=>'
}}}
I think it would be helpful to give the same suggestion as above.
-- Neil
Comment:
Good suggestion, but harder to implement. Without `ScopedTypeVariables`,
`forall` is not a keyword, so GHC is trying to parse that thing as a
'''type''', and it just doesn't follow the syntax for types.
The solution is probably this:
* Do not treat `=>` specially in the parser
* Instead parse it as an infix operator
* In the post-parsing re-organisation (`RdrHsSyn`), unswizzle the `=>`
into a constraint set
* But in doing so, if the stuff on the left of the `=>` doesn't look like
a constraint set, look for a leading `forall` and complain appropriately
Not really hard, but fiddly.
Simon
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2588#comment:1>
GHC <http://www.haskell.org/ghc/>
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