[Haskell] Types of when and unless in Control.Monad

[Andreas Abel]

On 06.06.12 12:13 AM, Henning Thielemann wrote:
>
> On Tue, 5 Jun 2012, Andres Löh wrote:
>
>>> Btw. the mapM memory leak can be easily and accidentally resurrected by
>>> writing
>>>
>>>  when_ b (mapM f xs)
>>>
>>> with
>>>
>>>  f :: a -> m ()
>>>  when_ :: Bool -> m a -> m ()
>>
>> I don't understand the whole mapM analogy (yet). I don't need when_ to
>> get into trouble with using mapM.
>
> If I write
>
> when b (mapM f xs)
>
> with
>
>   f :: a -> m ()
>   when :: Bool -> m () -> m ()
>
> then GHC will say something like: Cannot match [()] (result of 'mapM')
> with () (argument of 'when').

Well, but nothing in the type system prevents you from writing

   mapM f xs >> cont

or

   do mapM f xs
      cont

so you are not warned of a space leak in the common situations anyway, 
but you are responsible yourself.

I don't know why 'when' should be any different than '>>'.

Cheers,
Andreas


-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
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