Type error for function main

[Ross Paterson]

On Thu, Oct 16, 2003 at 02:35:26PM +0200, Josef Svenningsson wrote:
> I was just trying to write the smallest valid Haskell program. One try was
> the following:
> 
> > main = return 0

Much too long -- 9 characters are enough.

> But I get the following error message:
> ERROR "foo.hs":1 - Unresolved top-level overloading
> *** Binding             : main
> *** Outstanding context : Monad c
> 
> Why do I get this? According to the Haskell report a module where one
> doesn't specify the name should implicitly be called Main. And a Main
> module have a function main of type IO t for some t. So Hugs should be
> able to figure out that the monad should be IO in this case, at least
> according the Haskell report.

I'm not so sure.  It could well be argued from 4.5.5 that type inference
for this binding group leaves the type variable ambiguous, and that the
restriction on the type of main is supposed to be a later restriction.
(And Hugs does the Monomorphism Restriction differently anyway.)
Hugs already departs from the Report in not requiring that anonymous
modules define main (to cater for people who can't be bothered with
a module header for little things).  It would take a special case to
handle main, it's not clear it's necessary, and who else would care?