Does this function have a name?

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Paul Brauner

7 Apr 2017 7 Apr '17

8:42 a.m.

Hello Haskell Cafe, I had to write the following function and I was wondering if it (or its generalization to Alternative or Traversable) was exposed by some library under a different name: first [] = Nothing first (t:ts) = fmap (:ts) (s t) <|> fmap (t:) (first ts) Somehow it is to "traverse" as "any" is to "all". Cheers, Paul

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Paul Brauner

7 Apr 7 Apr

8:57 a.m.

Oops, there's a free variable in there, let me fix it: first f [] = Nothing first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs) Paul On Fri, Apr 7, 2017 at 10:42 AM Paul Brauner wrote:

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Hello Haskell Cafe,

I had to write the following function and I was wondering if it (or its generalization to Alternative or Traversable) was exposed by some library under a different name:

first [] = Nothing first (t:ts) = fmap (:ts) (s t) <|> fmap (t:) (first ts)

Somehow it is to "traverse" as "any" is to "all".

Cheers, Paul

Matt

6:53 p.m.

I think you're looking at the Monoid instance for First: λ> import Data.Monoid λ> let first f = getFirst . foldMap (First . f) λ> first (\x -> guard (x > 2) *> pure x) [1..10] Just 3 Matt Parsons On Fri, Apr 7, 2017 at 2:57 AM, Paul Brauner wrote:

...

Oops, there's a free variable in there, let me fix it:

first f [] = Nothing first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs)

Paul

On Fri, Apr 7, 2017 at 10:42 AM Paul Brauner wrote:

...
Hello Haskell Cafe,

I had to write the following function and I was wondering if it (or its generalization to Alternative or Traversable) was exposed by some library under a different name:

first [] = Nothing first (t:ts) = fmap (:ts) (s t) <|> fmap (t:) (first ts)

Somehow it is to "traverse" as "any" is to "all".

Cheers, Paul

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Paul Brauner

7:24 p.m.

Hi Matt, this isn't exactly the same thing I think. My function replaces the first element in the list for which f returns (Just x) by x, and leaves the rest of the list untouched. Paul On Fri, Apr 7, 2017, 20:53 Matt wrote:

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I think you're looking at the Monoid instance for First:

λ> import Data.Monoid λ> let first f = getFirst . foldMap (First . f) λ> first (\x -> guard (x > 2) *> pure x) [1..10] Just 3

Matt Parsons

On Fri, Apr 7, 2017 at 2:57 AM, Paul Brauner wrote:

Oops, there's a free variable in there, let me fix it:

first f [] = Nothing first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs)

Paul

On Fri, Apr 7, 2017 at 10:42 AM Paul Brauner wrote:

Hello Haskell Cafe,

I had to write the following function and I was wondering if it (or its generalization to Alternative or Traversable) was exposed by some library under a different name:

first [] = Nothing first (t:ts) = fmap (:ts) (s t) <|> fmap (t:) (first ts)

Somehow it is to "traverse" as "any" is to "all".

Cheers, Paul

_______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

John Wiegley

8 Apr 8 Apr

9:20 p.m.

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"PB" == Paul Brauner writes:

PB> Oops, there's a free variable in there, let me fix it: PB> first f [] = Nothing PB> first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs) If you break up the test from the modification (instead of fusing the operation into 'f'), this sort of thing becomes quite easy to express using lens: [1..10] & partsOf (traverse.filtered (>2))._head +~ 3 If the list has no elements >2, the operation is a no-op. -- John Wiegley GPG fingerprint = 4710 CF98 AF9B 327B B80F http://newartisans.com 60E1 46C4 BD1A 7AC1 4BA2

Paul Brauner

10 Apr 10 Apr

8:07 a.m.

Hi John, this is a nice way of expressing it! I was hoping there would be some more fundamental function that I would have missed though. By fundamental I mean that I was hoping there was some function of a "core" typeclass like Alternative that I had missed that was a general case of my function. Thanks anyway, I learned about lens' partsOf :) Paul On Sat, Apr 8, 2017 at 11:20 PM John Wiegley wrote:

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"PB" == Paul Brauner writes:

PB> Oops, there's a free variable in there, let me fix it: PB> first f [] = Nothing PB> first f (x:xs) = fmap (:xs) (f x) <|> fmap (x:) (first f xs)

If you break up the test from the modification (instead of fusing the operation into 'f'), this sort of thing becomes quite easy to express using lens:

[1..10] & partsOf (traverse.filtered (>2))._head +~ 3

If the list has no elements >2, the operation is a no-op.

-- John Wiegley GPG fingerprint = 4710 CF98 AF9B 327B B80F http://newartisans.com 60E1 46C4 BD1A 7AC1 4BA2

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John Wiegley
Matt
Paul Brauner