[Haskell-cafe] Badly designed Parsec combinators?

[Jan-Willem Maessen]

On Feb 16, 2006, at 7:32 PM, John Meacham wrote:

> ...
>>   Again that doesn't compile, because "when" requires a ()-returning
>> monad as its second parameter, but the "string" parser returns  
>> "String".
>> Same thing with if-then-else, when used to switch IO actions and  
>> such:
>> the IO actions must fully match in type, even if the returned  
>> value will
>> be discarded, and again that can be trivially resolved by adding the
>> "return ()".
>
> This is a straight up bug in the definition of when I hope we fix. it
> should have type
>
> when :: Bool -> IO a -> IO ()
> when = ...

Arguably this could be made true of *every* function which presently  
takes m () as an argument.  That is, we could systematically go  
through the libraries and convert every function of type:

f :: (Monad m) => .... -> m () -> ...

into

f :: (Monad m) => .... -> m otherwiseUnusedTypeVariable -> ...

This would basically eliminate the need for "ignore".  I can see  
taste arguments in either direction, but really the language ought to  
pick an alternative and use it everywhere (including for >>).

-Jan-Willem Maessen

>
>         John
>
>
> -- 
> John Meacham - ⑆repetae.net⑆john⑈
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