whenJust / forM_ (was Re: Proposal: Add isLeft/isRight to Data.Either)

Andreas Abel andreas.abel at ifi.lmu.de
Mon Dec 10 11:43:33 CET 2012

On 10.12.12 4:42 AM, Conrad Parker wrote:
> On 8 December 2012 02:38, Erik Hesselink <hesselink at gmail.com> wrote:
>> On Friday, December 7, 2012, Thomas Schilling wrote:
>>> On 7 December 2012 09:25, Erik Hesselink <hesselink at gmail.com> wrote:
>>>> There is no need to define whenJust, since it is just a specialization
>>>> of forM_ from Data.Foldable.
>>> I don't know if that's such a good argument.  I prefer my code
>>> readable -- too much abstraction can have a seriously bad effect on
>>> that.  As a simple rule, if the user has to do type inference in their
>>> head, readability suffers.
>>> In this case, "forM_" implies we're iterating over something container
>>> like, "whenJust" means "I want to do something if this thing yielded a
>>> result". It depends on the context whether a "Maybe" is behaving more
>>> container-like or is indicating the presence of a result where there
>>> may not have been one.
>> I don't have this problem, but I guess that's just personal. But in general
>> I'm not in favor of adding specialized versions of generalized functions.

At least in my mind, there is a conceptual distinction between a 
conditional execution such as whenJust and an iteration such as forM_. 
But I guess I am not alone in favoring, say,

   if (debugLevel >= 1) then print ("Bla")


   while (debugLevel >= 1) do print ("Bla")

So for purposes of program readability, whenJust has clearly a right of 
existence, even if it is just an alias for forM_.

> indeed, and likewise we should get rid of
> Control.Monad.forM_ :: Monad m => [a] -> (a -> m b) -> m ()
> which is just the same specialization to lists (and likely the reason
> that forM_ doesn't come to mind for other container types, as much as
> I generally like Data.Foldable).

This is a another issue, these special instances of mapM for lists are 
indeed annoying...

> btw:
> $ git grep whenJust|wc -l
> 40


Andreas Abel  <><      Du bist der geliebte Mensch.

Theoretical Computer Science, University of Munich
Oettingenstr. 67, D-80538 Munich, GERMANY

andreas.abel at ifi.lmu.de

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