[Haskell-beginners] forM vs mapM

Gaius Hammond Gaius at Gaius.org.UK
Thu Aug 19 06:16:06 EDT 2010

Thanks, I suspected that might be the case but it seemed too simple :-)




-----Original Message-----
From: Thomas Davie <tom.davie at gmail.com>
Date: Thu, 19 Aug 2010 11:08:34 
To: <Gaius at Gaius.org.UK>
Cc: Haskell Beginners List<beginners at haskell.org>
Subject: Re: [Haskell-beginners] forM vs mapM

On 19 Aug 2010, at 11:02, Gaius Hammond wrote:

> Hi all,
> Can anyone give me an example of a situation where you would use forM but *not* mapM or vice versa? As far as I can see one is simply a flip of the other. Does forM make any guarantees about order of execution?

Nope, no extra guarentees, just reverses the argument order.

forM can look quite nice if you have a large function that you want to map...

forM [someList]
  lots of
  large function

while mapM can look good if you have a small function...
mapM simpleFunction [someList]

They're also useful both ways round partially applied.


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