Apologies if my earlier message came across as a bit brutal. When I first saw this pattern a little while ago, I was surprised by it too! Cheers, Richard On Aug 9, 2015, at 11:55 PM, M Farkas-Dyck wrote:

On 09/08/2015 at 22:53:01 -0400, Richard Eisenberg wrote:

...

This can be done today:

...

allMinus m ns = [ n' | n <- ns, Just n' <- [n -? m] ]

Ah, obviously. Please excuse my earlier stupidity.

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 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 wrote:

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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? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

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On Sun, Aug 9, 2015 at 10:23 PM, Richard Eisenberg wrote:

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.

On a somewhat related note, this trick is a nice way to introduce let-like bindings in languages like Python that have comprehension syntax and no explicit support for pure intermediate bindings: [ str(x) + ' is ' + str(i) + ' dozens' for i in range(1, 9) for x in [12*i] ] It’s also useful when you want to simulate monadic-ish things sometimes: x = {'a': { 'b': 'c', 'd': 'e'}, 'f': 'g'} [ z for y in [x.get('a')] if y for z in [y.get('b')] if z ] # => ['c'] [ z for y in [x.get('q')] if y for z in [y.get('b')] if z ] # => [] Almost feels like having some simple monads!