Desugar list comprehension

Previous Topic Next Topic
 
classic Classic list List threaded Threaded
3 messages Options
Reply | Threaded
Open this post in threaded view
|

Desugar list comprehension

Mike Houghton
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





_______________________________________________
Beginners mailing list
[hidden email]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
Reply | Threaded
Open this post in threaded view
|

Re: Desugar list comprehension

David McBride
If you check on hoogle for how guard is written, it is just this
guard True  = pure ()
guard False = 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 <[hidden email]> 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




_______________________________________________
Beginners mailing list
[hidden email]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

_______________________________________________
Beginners mailing list
[hidden email]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
Reply | Threaded
Open this post in threaded view
|

Re: Desugar list comprehension

Mike Houghton

Duh!  Of course, thanks David.



On 2 Dec 2020, at 12:51, David McBride <[hidden email]> wrote:

If you check on hoogle for how guard is written, it is just this
guard True  = pure ()
guard False = 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 <[hidden email]> 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




_______________________________________________
Beginners mailing list
[hidden email]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
_______________________________________________
Beginners mailing list
[hidden email]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners


_______________________________________________
Beginners mailing list
[hidden email]
http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners