Re: [Haskell-cafe] [Maybe Int] sans Nothings
On Mon, May 23, 2011 at 6:21 PM, Richard O'Keefe o...@cs.otago.ac.nz wrote: On 24/05/2011, at 5:49 AM, Alexander Solla wrote: There's a library function for it, but also: filter ((/=) Nothing) The problem with that in general is that it only applies to [Maybe t] if Eq t, but you don't actually _need_ t to support equality. filter isJust will do the job, where isJust is in Data.Maybe. Indeed, isJust will do the job. Sometimes it is acceptable to just use the tools you know. For example, filtering a list of (Maybe Int)s. This is a balance that is tough to get right. Personally, I find non-functional values without Eq instances to be degenerate. So I really do not mind superfluous Eq constraints. I would not hesitate to use filter ((/=) Nothing) in a function whose type has no free type variables. It's just a bit of plumbing inside of a more complex function. But the point of avoiding unnecessary constraints is a good one, especially for constraints that constrain more strongly, e.g., Ord. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
Alexander Solla schrieb: Personally, I find non-functional values without Eq instances to be degenerate. So I really do not mind superfluous Eq constraints. I would not hesitate to use filter ((/=) Nothing) in a function whose type has no free type variables. It's just a bit of plumbing inside of a more complex function. Sometimes it seems to be better to not allow Eq on Float and Double. Since most algebraic laws do not hold for those types, it is more often an error than an intention to compare two Float values. And how to compare (IO a) values? Also, by thinking about function types, you often get interesting use cases. Thus I would not assume too quickly that a type will always be instantiated by types other than a function type. Thus I would stick to (filter isJust) and use this consistently for monomorphic and polymorphic types. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
Personally, I find non-functional values without Eq instances to be degenerate. So I really do not mind superfluous Eq constraints. I would not hesitate to use filter ((/=) Nothing) in a function whose type has no free type variables. It's just a bit of plumbing inside of a more complex function. Sometimes it seems to be better to not allow Eq on Float and Double. Since most algebraic laws do not hold for those types, it is more often an error than an intention to compare two Float values. And how to compare (IO a) values? Floats, Doubles, and IO are all degenerate types, for the reasons you outline. (Admittedly, Float and Double have Eq instances, but invalid Eq semantics) Notice how their value semantics each depend on the machine your runtime runs on, as opposed to merely the runtime. Bottom is another one of these degenerate types, since comparisons on arbitrary values are undecidable. Also, by thinking about function types, you often get interesting use cases. Thus I would not assume too quickly that a type will always be instantiated by types other than a function type. Thus I would stick to (filter isJust) and use this consistently for monomorphic and polymorphic types. I am not suggesting (filter ((/=) Nothing)) /over/ (filter isJust). Obviously, once one is aware of a better tool, one should use it. But I am suggesting that for simple cases which are unlikely to change in any substantive way, we should probably just use the tools we already know of, as opposed to searching for the right one. Both might involve costs. There is a cost involved in going to Google, thinking up a search term, finding that Data.Maybe has relevant functions, picking the right one. It takes less time to write filter ... than to type haskell removing nothing from list, for example. When dealing with known unknowns, there is a balance to be made, and it is not easy. Michael's choice to ask the list imposed costs. (Not that we mind, we're all volunteers, after all). But it probably took 10 minutes to get the first reply. He could have written a bit of code that worked correctly, given the context of his problem, in 20 seconds. Then again, he probably worked on a different bit of code until somebody sent a solution, so we probably only have to account for the time spent in context switching, for everyone involved. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] [Maybe Int] sans Nothings
What's the best way to end up with a list composed of only the Just values,no
Nothings?
Michael
==
import Control.Monad.Stateimport Data.Maybe
type GeneratorState = State Int
tick :: GeneratorState (Maybe Int)tick = do n - get if ((n `mod` 7)
== 0) then return Nothing else do
put (n+1) return (Just n)
{-*Main evalState (sequence $ replicate 9 tick) 1[Just 1,Just 2,Just 3,Just
4,Just 5,Just 6,Nothing,Nothing,Nothing]-}
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Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 23 May 2011, at 17:20, michael rice wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? Go to haskell.org/hoogle Type in [Maybe a] - [a] Click on first result. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 5/23/11 9:20 AM, michael rice wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? Try catMaybes in Data.Maybe. Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 23 May 2011 17:20, michael rice nowg...@yahoo.com wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? http://haskell.org/hoogle/?hoogle=%3A%3A+%5BMaybe+a%5D+-%3E+%5Ba%5D Data.Maybe.catMaybes is what you want :-) Cheers, Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 23 May 2011, at 17:20, michael rice wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? Alternatively, [ x | Just x - originals ] It also occurs to me that perhaps you still want the Just constructors. [ Just x | Just x - originals ] [ x | x@(Just _) - originals ] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 5/23/11 9:29 AM, Max Bolingbroke wrote: On 23 May 2011 17:20, michael rice nowg...@yahoo.com mailto:nowg...@yahoo.com wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? http://haskell.org/hoogle/?hoogle=%3A%3A+%5BMaybe+a%5D+-%3E+%5Ba%5D Data.Maybe.catMaybes is what you want :-) Cheers, Max On 5/23/11 9:25 AM, Malcolm Wallace wrote: On 23 May 2011, at 17:20, michael rice wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? Go to haskell.org/hoogle Type in [Maybe a] - [a] Click on first result. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe On 5/23/11 9:25 AM, Gregory Crosswhite wrote: On 5/23/11 9:20 AM, michael rice wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? Try catMaybes in Data.Maybe. Cheers, Greg GO TEAM HASKELL!!! Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On Mon, May 23, 2011 at 9:20 AM, michael rice nowg...@yahoo.com wrote:
What's the best way to end up with a list composed of only the Just values,
no Nothings?
Michael
==
import Control.Monad.State
import Data.Maybe
type GeneratorState = State Int
tick :: GeneratorState (Maybe Int)
tick = do n - get
if ((n `mod` 7) == 0)
then
return Nothing
else do
put (n+1)
return (Just n)
{-
*Main evalState (sequence $ replicate 9 tick) 1
[Just 1,Just 2,Just 3,Just 4,Just 5,Just 6,Nothing,Nothing,Nothing]
-}
There's a library function for it, but also:
filter ((/=) Nothing)
is readable enough.
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Re: [Haskell-cafe] [Maybe Int] sans Nothings
Thanks, all. Earlier, I was going to ask how to break out of a sequence op prematurely, i.e., you give it some replication number but want to break early if you get an end-flag value. While I was composing the post I thought of using Maybe for the good values and Nothing for the end value. Ergo, losing the Nothings at the end. I was passing a map and an empty list into some state, but since the map doesn't change once it's created I moved it into a Reader. I was using the list to collect the elements but thought I can get that functionality automatically using sequence. So, one thing leads to another. It's interesting how ideas begin bubbling up after one absorbs some critical mass of Haskell. Michael --- On Mon, 5/23/11, Malcolm Wallace malcolm.wall...@me.com wrote: From: Malcolm Wallace malcolm.wall...@me.com Subject: Re: [Haskell-cafe] [Maybe Int] sans Nothings To: haskell-cafe Cafe haskell-cafe@haskell.org Date: Monday, May 23, 2011, 12:35 PM On 23 May 2011, at 17:20, michael rice wrote: What's the best way to end up with a list composed of only the Just values, no Nothings? Alternatively, [ x | Just x - originals ] It also occurs to me that perhaps you still want the Just constructors. [ Just x | Just x - originals ] [ x | x@(Just _) - originals ] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On Mon, May 23, 2011 at 10:49:55AM -0700, Alexander Solla wrote:
On Mon, May 23, 2011 at 9:20 AM, michael rice nowg...@yahoo.com wrote:
What's the best way to end up with a list composed of only the Just values,
no Nothings?
Michael
==
import Control.Monad.State
import Data.Maybe
type GeneratorState = State Int
tick :: GeneratorState (Maybe Int)
tick = do n - get
if ((n `mod` 7) == 0)
then
return Nothing
else do
put (n+1)
return (Just n)
{-
*Main evalState (sequence $ replicate 9 tick) 1
[Just 1,Just 2,Just 3,Just 4,Just 5,Just 6,Nothing,Nothing,Nothing]
-}
There's a library function for it, but also:
filter ((/=) Nothing)
is readable enough.
Just a minor quibble: note that
filter (not . isNothing)
is slightly preferable since it does not introduce a frivolous
equality constraint on the type wrapped by the Maybe.
-Brent
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Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 05/23/2011 12:08 PM, Brent Yorgey wrote: Just a minor quibble: note that filter (not . isNothing) is slightly preferable since it does not introduce a frivolous equality constraint on the type wrapped by the Maybe. Or even better, filter isJust :-) Cheers, Greg ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
Brent Yorgey schrieb: On Mon, May 23, 2011 at 10:49:55AM -0700, Alexander Solla wrote: There's a library function for it, but also: filter ((/=) Nothing) is readable enough. Just a minor quibble: note that filter (not . isNothing) is slightly preferable since it does not introduce a frivolous equality constraint on the type wrapped by the Maybe. Similar: http://www.haskell.org/haskellwiki/Haskell_programming_tips#Reduce_type_class_constraints http://www.haskell.org/haskellwiki/Haskell_programming_tips#Don.27t_ask_for_the_length_of_a_list_when_you_don.27t_need_it ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
Gregory Crosswhite gcr...@phys.washington.edu wrote: Or even better, filter isJust To make it worse again the original function can be generalized in a few ways. Here is a generalization from the inner Maybe type: import Data.Foldable as F catFoldables :: Foldable t = [t a] - [a] catFoldables = concatMap F.toList Here is a generalization from the outer list type: joinMaybes :: (Alternative m, Monad m) = m (Maybe a) - m a joinMaybes = (= maybe empty pure) And finally the generalization from everything: import Data.Foldable as F joinFoldables :: (Alternative m, Foldable t, Monad m) = m (t a) - m a joinFoldables = (= F.foldr (\x _ - pure x) empty) The final function looks a bit scary, but is actually surprisingly easy to understand, once you realize that 'foldr' is just a generalization of the 'maybe' function. The structure of Maybe is a list structure with at most one element after all. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife = sex) http://ertes.de/ -- nightmare = unsafePerformIO (getWrongWife = sex) http://ertes.de/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Maybe Int] sans Nothings
On 24/05/2011, at 5:49 AM, Alexander Solla wrote: There's a library function for it, but also: filter ((/=) Nothing) The problem with that in general is that it only applies to [Maybe t] if Eq t, but you don't actually _need_ t to support equality. filter isJust will do the job, where isJust is in Data.Maybe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
