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Today's Topics:
1. Re: Desugar list comprehension (David McBride)
2. Re: Desugar list comprehension (mike h)
----------------------------------------------------------------------
Message: 1
Date: Wed, 2 Dec 2020 07:51:40 -0500
From: David McBride <toa...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Desugar list comprehension
Message-ID:
<CAN+Tr42Mx0XWDXg9T-FyWD_r5+K-7dCookK=ZkqwizVmxw64=a...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
If you check on hoogle for how guard is written, it is just this
guard True = pure
<https://hackage.haskell.org/package/base-4.14.0.0/docs/Control-Applicative.html#v:pure>
()
guard False = empty
<https://hackage.haskell.org/package/base-4.14.0.0/docs/Control-Applicative.html#v:empty>
That means you can use the same thing in your own code
import Control.Applicative
pairs xs =
xs >>= \x ->
xs >>= \y ->
if (x + y == 2020) then pure (x,y) else empty
On Wed, Dec 2, 2020 at 5:31 AM mike h <mike_k_hough...@yahoo.co.uk> wrote:
> Hi,
> I have
> sumIs2020P1' xs = do
> x <- xs
> y <- xs
> guard (x + y == 2020)
> pure (x,y)
>
> which has been desugared from a list comprehension
> I would like to reduce this even more using >>=
> So I do
> sumIs2020P1'' xs = (a,b) where
> (a,b):rest = filter (\(x,y) -> x + y == 2020) pairs
>
> pairs = xs >>= \x ->
> xs >>= \y ->
> pure (x,y)
>
> but really I would like the guard to be within the >>= sections but I
> could not work out
> how to do it!
> i.e. I’m looking for something like (pseudo code)
>
> pairs = xs >>= \x ->
> xs >>= \y ->
> if (x + y == 2020) then pure (x,y) else DO_NOTHING
> which would then allow the filter to be removed.
>
>
> Many Thanks
>
> Mike
>
>
>
> Dr Mike Houghton
>
> mike_k_hough...@yahoo.co.uk
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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Message: 2
Date: Wed, 2 Dec 2020 15:47:05 +0000
From: mike h <mike_k_hough...@yahoo.co.uk>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] Desugar list comprehension
Message-ID: <75d621d3-e4dd-4777-9806-5be28a375...@yahoo.co.uk>
Content-Type: text/plain; charset="utf-8"
Duh! Of course, thanks David.
> On 2 Dec 2020, at 12:51, David McBride <toa...@gmail.com> wrote:
>
> If you check on hoogle for how guard is written, it is just this
> guard True = pure
> <https://hackage.haskell.org/package/base-4.14.0.0/docs/Control-Applicative.html#v:pure>
> ()
> guard False = empty
> <https://hackage.haskell.org/package/base-4.14.0.0/docs/Control-Applicative.html#v:empty>
> That means you can use the same thing in your own code
>
> import Control.Applicative
>
> pairs xs =
> xs >>= \x ->
> xs >>= \y ->
> if (x + y == 2020) then pure (x,y) else empty
>
> On Wed, Dec 2, 2020 at 5:31 AM mike h <mike_k_hough...@yahoo.co.uk
> <mailto:mike_k_hough...@yahoo.co.uk>> wrote:
> Hi,
> I have
> sumIs2020P1' xs = do
> x <- xs
> y <- xs
> guard (x + y == 2020)
> pure (x,y)
>
> which has been desugared from a list comprehension
> I would like to reduce this even more using >>=
> So I do
> sumIs2020P1'' xs = (a,b) where
> (a,b):rest = filter (\(x,y) -> x + y == 2020) pairs
>
> pairs = xs >>= \x ->
> xs >>= \y ->
> pure (x,y)
>
> but really I would like the guard to be within the >>= sections but I could
> not work out
> how to do it!
> i.e. I’m looking for something like (pseudo code)
>
> pairs = xs >>= \x ->
> xs >>= \y ->
> if (x + y == 2020) then pure (x,y) else DO_NOTHING
>
> which would then allow the filter to be removed.
>
>
> Many Thanks
>
> Mike
>
>
>
> Dr Mike Houghton
>
> mike_k_hough...@yahoo.co.uk <mailto:mike_k_hough...@yahoo.co.uk>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org <mailto:Beginners@haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
> <http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
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