Hi,
Today, when I was restructuring my program, I discovered something nasty
in one of Hugs' error messages. Simplified, it looked as follows:
ERROR "Main.hs" (line 6): Type error in application
*** expression : useFoo mkFoo
*** term : mkFoo
*** type : Foo
*** does not match : Foo
????? I was puzzled!
Later, it appeared I had made a mistake in renaming my modules, because my
system had looked as follows:
module Baz( mkFoo ) where
data Foo = Foo
mkFoo :: Foo
mkFoo = Foo
module Bar( useFoo ) where
data Foo = Foo
useFoo :: Foo -> Int
useFoo Foo = 3
module Main where
import Baz
import Bar
main = useFoo mkFoo
I had wished Hugs' error message to be something like:
ERROR "Main.hs" (line 6): Type error in application
*** expression : useFoo mkFoo
*** term : mkFoo
*** type : Baz.Foo
*** does not match : Bar.Foo
This brings us to another issue: what does it really mean when you export
a function from a module, but not the name of one of the types used in the
function type of that function?
Maybe we should not allow programmers to export a function whose type is
not completely known outside that module as well. If I had been forced
to write:
module Baz( mkFoo, Foo ) where
...
module Bar( useFoo, Foo ) where
...
Hugs would have told me:
ERROR "Main.hs": Entity "Foo" imported from module "Baz" already defined
in module "Bar"
which makes clear what the problem is.
Regards,
Koen.
--
Koen Claessen,
[EMAIL PROTECTED],
http://www.cs.chalmers.se/~koen,
Chalmers University of Technology.
