[Haskell-cafe] for = flip map

[Thomas Schilling]

On 29 March 2012 22:03, Sjoerd Visscher <sjoerd at w3future.com> wrote:
> Some more bikeshedding:
>
> Perhaps ffor, as in
>
>    ffor = flip fmap
>
> or perhaps
>
>    infixr 0 <$$>
>    (<$$>) = flip (<$>)
>
>    xs <$$> \x -> ...

I don't think it makes sense to add a whole new operator for that.
You can just use sections:

  (<$> xs) \x -> ...

The reason you can't do this with <*> is the ordering of effects.

I have to admit, though, that the above isn't exactly readable.  The
non-operator version is somewhat more readable:

  (`map` xs) \x -> ...

I'd still prefer "for" or "foreach".

>
> (cf. <**>)
>
> In both cases they should go in Data.Functor
>
> Sjoerd
>
> On Mar 28, 2012, at 11:26 PM, ezra at ezrakilty.net wrote:
>
>> I would very much like to see a standard function for "flip map" along
>> these lines. I think it would make a lot of code more readable.
>>
>> Like the OP, I use "for" in my own code. It's unfortunate that
>> Data.Traversable takes the name with another type. Two options would be
>> to (a) reuse the name in Data.List and force people to qualify as
>> necessary, or (b) choose another name for "flip map".
>>
>> Regarding other possible names: forall is a keyword and forAll is used
>> by QuickCheck. One possibility would be "foreach".
>>
>> Ezra
>>
>> On Wed, Mar 28, 2012, at 10:19 PM, Christopher Done wrote:
>>> On 28 March 2012 22:05, Matthew Steele <mdsteele at alum.mit.edu> wrote:
>>>> Doesn't for already exist, in Data.Traversable?   Except that for =
>>>> flip traverse.
>>>
>>> Traverse doesn't fit the type of fmap, it demands an extra type
>>> constructor:
>>>
>>> traverse :: (Traversable t,Applicative f) => (a -> f b) -> t a -> f (t b)
>>>
>>> fmap :: Functor f => (a -> b) -> f a -> f b
>>>
>>> Note the (a -> f b) instead of (a -> b).
>>>
>>> E.g.
>>>
>>> fmap :: (a -> b) -> [a] -> [b]
>>>
>>> can't be expressed with traverse, you can only get this far:
>>>
>>> traverse :: (a -> [b]) -> [a] -> [[b]]
>>>
>>> Unless I'm missing something.
>>>
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>>
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>
> --
> Sjoerd Visscher
> https://github.com/sjoerdvisscher/blog
>
>
>
>
>
>
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