[Haskell-cafe] Pattern guards in comprehensions

[amindfv at gmail.com]

There's an even nicer way! Use view patterns:

allMinus m ns = [ n' | ((?- m) -> Just n') <- ns ]

I wrote a blog post about this when i discovered it but my blog's not up at the moment...

Tom

El Aug 9, 2015, a las 22:53, Richard Eisenberg <eir at cis.upenn.edu> escribió:

> This can be done today:
> 
>> allMinus m ns = [ n' | n <- ns, Just n' <- [n -? m] ]
> 
> The syntax is a bit regrettable, but it works quite well. You could use `Just n' <- return (n -? m)` if you wanted, or be even more pedantic and make a synonym `patternMatch :: a -> [a]; patternMatch = return` and say `Just n' <- patternMatch (n -? m)`.
> 
> Richard
> 
> On Aug 9, 2015, at 8:48 PM, M Farkas-Dyck <strake888 at gmail.com> wrote:
> 
>> I propose we add pattern guards in comprehensions, but I'm not sure what syntax would work. Consider this function:
>> 
>> allMinus :: [Natural] -> [Natural]
>> allMinus m = mapMaybe (-? m)
>> where n -? m | m > n = Nothing
>>                    | True  = Just (n-m)
>> 
>> One may wish to write such as a list comprehension, but it is cumbersome:
>> 
>> allMinus m ns = [n' | n@((-? m) -> Just n') <- ns]
>> 
>> This would be clearer with pattern guards, fictitious syntax here:
>> 
>> allMinus m ns = [n' | n <- ns, Just n' <- n -? m]
>> 
>> Alas, this conflicts with the other part of list comprehension syntax. Try we this, actual syntax now:
>> 
>> allMinus m ns = [n' | n <- ns, let Just n' = n -? m]
>> 
>> Nope, that's an error if (any (< m) ns).
>> 
>> I recognize in this case the one in terms of mapMaybe is quite clear, but in the case of some other code I'm writing it's much more complicated.
>> 
>> Ideas:
>> 1. Modify semantics of let in comprehension to skip that element on pattern mismatch
>> 2. Use another keyword, e.g. [n' | n <- ns where Just n' <- n -? m]
>> 
>> Thoughts?
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