[Haskell-beginners] best way to code this

[Alex Hammel]

f as alist = [ b | (a, b) <- alist, a `elem` as ]

perhaps?

On Sat, 30 May 2015 6:47 pm Lyndon Maydwell <maydwell at gmail.com> wrote:

> I think you're looking for `mapMaybe` :-)
>
>
>  - Lyndon
>
> On Sun, May 31, 2015 at 11:30 AM, <briand at aracnet.com> wrote:
>
>> Hi,
>>
>> A simple example of something I was trying to do but it had some
>> questions along the lines of "the best way to do this".
>>
>> Given a list
>>
>>   l = [a]
>>
>> and an a-list
>>
>>   alist = [ (a,b) ]
>>
>> the idea is to find all the items in l which are in alist and then create
>> a list [b]
>>
>> so the overall function should be
>>
>>  [a] -> [ (a,b) ] -> [b]
>>
>> the solution is straightforward:
>>
>>  l1 = filter (\x -> isJust (lookup x alist)) l
>>  l2 = map (\x -> fromJust (lookup x alist)) l1
>>
>> `fromJust` used in the construction of l2 won't fail, because only the
>> elements for which the lookup succeeded are in l1.
>>
>> This would be something called `filterMap` but I couldn't find such a
>> function in the list library, but it seems like there just has to be one
>> defined in a library somewhere.
>>
>> the above seems clumsy, i'm wondering how to make it "more pretty".
>>
>> generally i was also wondering if the above construction is as
>> inefficient as it looks because of the double-lookup, or would the compiler
>> actually be able to optimize that code into something more efficient ?
>> this code is not being used on large sets of data so efficiency doesn't
>> matter, I'm just curious.
>>
>> Thanks,
>>
>> Brian
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>
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