Thanks for a fine bug report.
Really there are two bugs
Bug 1: without -fglasgow-exts, this program should be rejected,
since Haskell 98 doesn't allow records with polymorphic fields.
Your record is really
data Record = R { blub :: forall a. Foo a => a -> [a] }
Bug 2: Assuming -fglasgow-exts, what type should blub have?
The "obvious" type is:
blub :: Record -> forall a. Foo a => a -> [a]
but GHC doesn't expect functions to have types with a forall
after a function arrow. This is isomorphic to
blub :: forall a. Record -> Foo a => a -> [a]
and that is in fact the type that GHC gives it. But having
given it that type, GHC trips up because it doesn't expect
to see types with a '=>' that isn't preceded by a 'forall'.
Hence the strange error message.
If we wrote the records selector in plain Haskell thus:
foo (R x) = x
then GHC would infer the type
foo :: forall a. Foo a => Record -> a -> [a]
and that would be the right type to give blub. I'll fix this in the HEAD.
And I hope we'll back-patch the fix to 4.08.2
Thanks again for an excellent report.
Simon
| -----Original Message-----
| From: Volker Stolz [mailto:[EMAIL PROTECTED]]
| Sent: 13 December 2000 08:29
| To: [EMAIL PROTECTED]
| Subject: Type checker too lazy
|
|
| Consider the following record (yes, I know, we all hate records but
| I use them frequently for read-only data):
|
| > import IO
| >
| > class Foo a where
| > bar :: a -> [a]
| >
| > instance Foo Int where
| > bar x = replicate x x
| >
| > data Record = R {
| > blub :: Foo a => a -> [a]
| > }
| >
| > main = do
| > let r = R {blub = bar}
| >
| > print (bar (3::Int)) -- works!
| > --print ((blub r) (3::Int)) -- doesn?t compile!
|
| As long as I don?t try to use "r", everything works fine.
| Using the latter
| print-statement, ghc yields
|
| classTest.lhs:14:
| Couldn't match `Int' against `{Foo a}'
| Expected type: Int
| Inferred type: {Foo a}
| In an expression with a type signature: 3 :: Int
|
| Hugs (Feb?00) is much more eager and wouldn?t even load the
| working case:
| ERROR "classTest.lhs" (line 10): Undefined type variable "a".
|
| Shouldn?t ghc reject the record-declaration altogether like
| Hugs? Why can?t
| I use the record like I?m doing in the first place?
| --
| \usepackage[latin1]{inputenc}!
| Volker Stolz * [EMAIL PROTECTED] * PGP + S/MIME
|
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