The simplest program

[Wolfgang Thaller]

Axel Simon wrote:

> This bug is older! ghc 5.04 does not enforce that the return type of 
> main
> is (). In fact with 5.04 you can return anything you want and get very
> fancy exception on termination if you return a more complex data type.

I don't know about an exception on termination (Is it a known bug? If 
not, it should be reported), but returning anything is allowed. The 
Haskell Report, section 5, says:

> A Haskell program is a collection of modules, one of which, by 
> convention, must be called Main and must export the value main. The 
> value of the program is the value of the identifier main in module 
> Main, which must be a computation of type IO t for some type t (see 
> Section 7). When the program is executed, the computation main is 
> performed, and its result (of type t) is discarded.

The problem with that no-op program is that the type checker doesn't 
seem to know that main has to be in the IO monad.
On the other hand...

 > module Main where
 > main :: Monad m => m ()
 > main = return ()

is legal according to both GHC 5.04 and 5.05. Now I'm beginning to see 
why GHC complains.
It would be nice, though, if "main = return ()" was legal without 
additional type signatures. I've been bitten by this before: I wrote 
some datatypes and functions in a "Main" module, but I left the main 
action empty at first. Then I just wanted to check syntax and typing 
and got a strange error message...

Cheers,

Wolfgang