Add ifThenElse and (?) to Data.Bool

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winter

17 Nov 2016 17 Nov '16

2:43 a.m.

It seems this’s a very old request, see https://wiki.haskell.org/If-then-else https://wiki.haskell.org/If-then-else. I’d like to see following: ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = x ifThenElse False _ y = y infixr 1 ? (?) :: Bool -> a -> a -> a (?) = ifThenElse in Date.Bool module, it will have advantages that: + It’s more composable than syntax. + Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes. + In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior. Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with. Cheers~ Winter

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David Feuer

17 Nov 17 Nov

3:03 a.m.

If ifThenElse is good for RebindableSyntax, then I'm +1 on that (but I've never played with that extension, so I don't really know). I'm -1 on (?). We already have bool, which tends to be rather more useful when partially applied. On Nov 16, 2016 9:43 PM, "winter" wrote:

...

It seems this’s a very old request, see https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = xifThenElse False _ y = y

infixr 1 ?(?) :: Bool -> a -> a -> a(?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax.

+ Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes.

+ In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~

Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

winter

3:16 a.m.

I’m totally aware of the existence of bool, i suppose (?) is mainly used in fully application to get a different style than if-then-else syntax, say, ... isGoo <- checkGoo isGoo ? goo $ woo ... But like what the wiki suggested, (?) can be used in some high-order situations. I like this operator because the mnemonic of questioning meaning.

...

On 17 Nov 2016, at 11:03, David Feuer wrote:

If ifThenElse is good for RebindableSyntax, then I'm +1 on that (but I've never played with that extension, so I don't really know). I'm -1 on (?). We already have bool, which tends to be rather more useful when partially applied.

On Nov 16, 2016 9:43 PM, "winter" mailto:drkoster@qq.com> wrote: It seems this’s a very old request, see https://wiki.haskell.org/If-then-else https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = x ifThenElse False _ y = y

infixr 1 ? (?) :: Bool -> a -> a -> a (?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax. + Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes. + In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~ Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org mailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

winter

4:01 a.m.

And here’s some other stuff i can came up with it(without obscured readability IMHO): ... -- replacement for ifM in various package, similar to (>>= when) (mFlag >>= (?)) flagEnabled -- mFlag :: Monad m => m Bool $ flagDisabled -- nicer if-then-else in applicative style (?) <$> fFlag <$> flagEnabled <*> flagDisabled -- compose with predicates to define your own if ifLower = (?) . isLower ifLower ‘X’ lower upper ... Basically it's a good if-then-else replacement if you’re comfortable with point-free style.

...

On 17 Nov 2016, at 11:16, winter wrote:

I’m totally aware of the existence of bool, i suppose (?) is mainly used in fully application to get a different style than if-then-else syntax, say,

... isGoo <- checkGoo isGoo ? goo $ woo ...

But like what the wiki suggested, (?) can be used in some high-order situations. I like this operator because the mnemonic of questioning meaning.

...
On 17 Nov 2016, at 11:03, David Feuer mailto:david.feuer@gmail.com> wrote:

If ifThenElse is good for RebindableSyntax, then I'm +1 on that (but I've never played with that extension, so I don't really know). I'm -1 on (?). We already have bool, which tends to be rather more useful when partially applied.

On Nov 16, 2016 9:43 PM, "winter" mailto:drkoster@qq.com> wrote: It seems this’s a very old request, see https://wiki.haskell.org/If-then-else https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = x ifThenElse False _ y = y

infixr 1 ? (?) :: Bool -> a -> a -> a (?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax. + Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes. + In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~ Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org mailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Edward Kmett

4:03 a.m.

(?) <$> fFlag <$> flagEnabled <*> flagDisabled You probably don't want this. This performs both effects (!), regardless of the flag, but only keeps one result. -Edward On Wed, Nov 16, 2016 at 11:01 PM, winter wrote:

...

And here’s some other stuff i can came up with it(without obscured readability IMHO):

... -- replacement for ifM in various package, similar to (>>= when)

(mFlag >>= (?)) flagEnabled -- mFlag :: Monad m => m Bool $ flagDisabled

-- nicer if-then-else in applicative style (?) <$> fFlag <$> flagEnabled <*> flagDisabled

-- compose with predicates to define your own if ifLower = (?) . isLower ifLower ‘X’ lower upper ...

Basically it's a good if-then-else replacement if you’re comfortable with point-free style.

On 17 Nov 2016, at 11:16, winter wrote:

I’m totally aware of the existence of bool, i suppose (?) is mainly used in fully application to get a different style than if-then-else syntax, say,

... isGoo <- checkGoo isGoo ? goo $ woo ...

But like what the wiki suggested, (?) can be used in some high-order situations. I like this operator because the mnemonic of questioning meaning.

On 17 Nov 2016, at 11:03, David Feuer wrote:

If ifThenElse is good for RebindableSyntax, then I'm +1 on that (but I've never played with that extension, so I don't really know). I'm -1 on (?). We already have bool, which tends to be rather more useful when partially applied.

On Nov 16, 2016 9:43 PM, "winter" wrote:

...
It seems this’s a very old request, see https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = xifThenElse False _ y = y

infixr 1 ?(?) :: Bool -> a -> a -> a(?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax.

+ Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes.

+ In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~

Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

winter

4:11 a.m.

Ah Yeah, i see. It’s indeed a rare use case. From all your feedback, I guess i will make a package with (?) for someone who want to save keystrokes like me ;) Cheers~ Winter

...

On 17 Nov 2016, at 12:03, Edward Kmett wrote:

(?) <$> fFlag <$> flagEnabled <*> flagDisabled

You probably don't want this. This performs both effects (!), regardless of the flag, but only keeps one result.

-Edward

On Wed, Nov 16, 2016 at 11:01 PM, winter mailto:drkoster@qq.com> wrote: And here’s some other stuff i can came up with it(without obscured readability IMHO):

... -- replacement for ifM in various package, similar to (>>= when)

(mFlag >>= (?)) flagEnabled -- mFlag :: Monad m => m Bool $ flagDisabled

-- nicer if-then-else in applicative style (?) <$> fFlag <$> flagEnabled <*> flagDisabled

-- compose with predicates to define your own if ifLower = (?) . isLower ifLower ‘X’ lower upper ...

Basically it's a good if-then-else replacement if you’re comfortable with point-free style.

...
On 17 Nov 2016, at 11:16, winter mailto:drkoster@qq.com> wrote:

I’m totally aware of the existence of bool, i suppose (?) is mainly used in fully application to get a different style than if-then-else syntax, say,

... isGoo <- checkGoo isGoo ? goo $ woo ...

But like what the wiki suggested, (?) can be used in some high-order situations. I like this operator because the mnemonic of questioning meaning.

...
On 17 Nov 2016, at 11:03, David Feuer mailto:david.feuer@gmail.com> wrote:

If ifThenElse is good for RebindableSyntax, then I'm +1 on that (but I've never played with that extension, so I don't really know). I'm -1 on (?). We already have bool, which tends to be rather more useful when partially applied.

On Nov 16, 2016 9:43 PM, "winter" mailto:drkoster@qq.com> wrote: It seems this’s a very old request, see https://wiki.haskell.org/If-then-else https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = x ifThenElse False _ y = y

infixr 1 ? (?) :: Bool -> a -> a -> a (?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax. + Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes. + In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~ Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org mailto:Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Henning Thielemann

8:36 a.m.

On Thu, 17 Nov 2016, winter wrote:

...

Ah Yeah, i see. It’s indeed a rare use case. From all your feedback, I guess i will make a package with (?) for someone who want to save keystrokes like me ;)

I use if' and (?:) from https://hackage.haskell.org/package/utility-ht-0.0.12/docs/Data-Bool-HT.html E.g. this is very handy: if' (even n) "even" $ if' (isPrime n) "prime" $ "boring" (?:) is the uncurried variant of (?). I do not see value in curried (?) because I can just use `if'` then (which I never did).

M Farkas-Dyck

3:06 a.m.

Could we use `bool` rather than add a new term for RebindableSyntax? i.e. define if-then-else in terms of `bool`.

David Feuer

3:25 a.m.

If this rebindable syntax is about what I think it is, that would seem strange. bool is the Bool eliminator. It would seem weird to give it a type like, say, bool :: Monad m => m a -> m a -> m Bool -> m a On Nov 16, 2016 10:06 PM, "M Farkas-Dyck" wrote:

...

Could we use `bool` rather than add a new term for RebindableSyntax? i.e. define if-then-else in terms of `bool`. _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Ivan Lazar Miljenovic

3:49 a.m.

I'm -0.5 on ifThenElse (if it can be used with RebindableSyntax then someone may want to use it, though I personally find the bool function to be more than sufficient as I tend to find the "which branch do I choose" argument to be the last when constructing function pipelines), and -1 for the operator. On 17 November 2016 at 13:43, winter wrote:

...

It seems this’s a very old request, see https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = x ifThenElse False _ y = y

infixr 1 ? (?) :: Bool -> a -> a -> a (?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax.

+ Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes.

+ In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~

Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

-- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com

Edward Kmett

4:02 a.m.

I'm pretty strongly -1 on adding (?). It is one of the few single character operators available to the average user out of the box and this is a space where we already have established combinators. It is a valuable portion of the namespace to spend and each approach we offer means more inessential complexity to newcomers to the language. Given the existence of bool today I'm weakly -1 on ifThenElse. That said, if we were to add RebindableSyntax support for it, I think that I'd personally flip around to being in favor. It is a much more clear thing for RebindableSyntax to call out to than something called "bool" that comes with a different argument order. These are just my personal feelings on the matter, and not any sort of "cast in stone" CLC judgments. -Edward On Wed, Nov 16, 2016 at 9:43 PM, winter wrote:

...

It seems this’s a very old request, see https://wiki.haskell.org/If-then-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = xifThenElse False _ y = y

infixr 1 ?(?) :: Bool -> a -> a -> a(?) = ifThenElse

in Date.Bool module, it will have advantages that:

+ It’s more composable than syntax.

+ Write (xxx ? yyy $ zzz) instead of (if xxx then yyy else zzz) is more consistent with (f . g $ x) style, and save key strokes.

+ In module with RebindableSyntax enabled, you can import ifThenElse to get default behavior.

Whether or not to be exported by Prelude is another question, but Data.Bool seems a good place to start with.

Cheers~

Winter

_______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Yitzchak Gale

10:14 a.m.

Edward Kmett wrote:

...

I'm pretty strongly -1 on adding (?)... we already have established combinators... more inessential complexity to newcomers to the language.

I'm not so worried about that if it's not in the Prelude. I doubt that this would get wide usage. And if it does, then maybe that is better use of the namespace real estate. I am 0 on (?). I probably would never use it. I am -1 on ifThenElse. We already have bool. Some of our developers use it all the time and it's fine. It's not like "map" and "for" which have common idioms where parameter order really matters. Other than that, we shouldn't litter the libraries with flipped versions of everything. Anyway, bool is the natural parameter order in Haskell. I'll make more explicit what others have already said, with this analogy: maybe (if it fails) (if it succeeds) (input data) either (if it fails) (if it succeeds) (input data) foldl (if it's null) (if it's not null) (input data) foldr (if it's null) (if it's not null) (input data) Whereas Haskell's if-then-else syntax is awkward. Some people (not me) even say it should never be used at all. And so bool (if it fails) (if it succeeds) (input data) makes the most sense. -Yitz

Henning Thielemann

10:19 a.m.

On Thu, 17 Nov 2016, Yitzchak Gale wrote:

...

Anyway, bool is the natural parameter order in Haskell. I'll make more explicit what others have already said, with this analogy:

maybe (if it fails) (if it succeeds) (input data) either (if it fails) (if it succeeds) (input data)

...

foldl (if it's null) (if it's not null) (input data) foldr (if it's null) (if it's not null) (input data)

Unfortunately, it is the other way round: foldl (if it's not null) (if it's null) (input data) foldr (if it's not null) (if it's null) (input data)

winter

12:19 p.m.

I decide to make a standalone package: http://hackage.haskell.org/package/if http://hackage.haskell.org/package/if Cheers! Winter

...

On 17 Nov 2016, at 18:19, Henning Thielemann wrote:

On Thu, 17 Nov 2016, Yitzchak Gale wrote:

...
Anyway, bool is the natural parameter order in Haskell. I'll make more explicit what others have already said, with this analogy:

maybe (if it fails) (if it succeeds) (input data) either (if it fails) (if it succeeds) (input data)

...
foldl (if it's null) (if it's not null) (input data) foldr (if it's null) (if it's not null) (input data)

Unfortunately, it is the other way round:

foldl (if it's not null) (if it's null) (input data) foldr (if it's not null) (if it's null) (input data) _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

Elliot Cameron

2:41 p.m.

FWIW, Winter, I sorta like that operator and it is even readable to people who use C, JS, etc. Thanks for the library. :) ᐧ On Thu, Nov 17, 2016 at 7:19 AM, winter wrote:

...

I decide to make a standalone package: http://hackage. haskell.org/package/if

Cheers! Winter

On 17 Nov 2016, at 18:19, Henning Thielemann < lemming@henning-thielemann.de> wrote:

On Thu, 17 Nov 2016, Yitzchak Gale wrote:

Anyway, bool is the natural parameter order in Haskell. I'll make more explicit what others have already said, with this analogy:

maybe (if it fails) (if it succeeds) (input data) either (if it fails) (if it succeeds) (input data)

foldl (if it's null) (if it's not null) (input data) foldr (if it's null) (if it's not null) (input data)

Unfortunately, it is the other way round:

foldl (if it's not null) (if it's null) (input data) foldr (if it's not null) (if it's null) (input data) _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Joachim Breitner

21 Nov 21 Nov

6:29 p.m.

Hi, Am Donnerstag, den 17.11.2016, 10:43 +0800 schrieb winter:

...

It seems this’s a very old request, see https://wiki.haskell.org/If-t hen-else. I’d like to see following:

ifThenElse :: Bool -> a -> a -> a ifThenElse True x _ = x ifThenElse False _ y = y

+1 on that. It is strange that GHC desugars to ifThenElse¹ without this function being available anywhere. Most (all?) of the other rebindable syntax elements work out of the box as before with Prelude imported but this one. Therefore, I’d support a proposal to add ifThenElse to the prelude. It would also make teaching nicer, by pointing students to this function and saying „if then else is just syntactic sugar for it“. Slightly better than „if then else is just syntactic sugar for a hypothetical function that you can define, but that is not there.“

...

infixr 1 ? (?) :: Bool -> a -> a -> a (?) = ifThenElse

-1. Operators are just too scarce. Greetings, Joachim ¹ http://downloads.haskell.org/~ghc/8.0.1/docs/html/users_guide/glasgow _exts.html#rebindable-syntax-and-the-implicit-prelude-import -- Joachim “nomeata” Breitner mail@joachim-breitner.de • https://www.joachim-breitner.de/ XMPP: nomeata@joachim-breitner.de • OpenPGP-Key: 0xF0FBF51F Debian Developer: nomeata@debian.org

Henning Thielemann

6:33 p.m.

On Mon, 21 Nov 2016, Joachim Breitner wrote:

...

It would also make teaching nicer, by pointing students to this function and saying „if then else is just syntactic sugar for it“. Slightly better than „if then else is just syntactic sugar for a hypothetical function that you can define, but that is not there.“

One implementation is there: http://hackage.haskell.org/package/utility-ht-0.0.12/docs/Data-Bool-HT.html#... if you want to point the students somewhere. :-)

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participants (9)

David Feuer
Edward Kmett
Elliot Cameron
Henning Thielemann
Ivan Lazar Miljenovic
Joachim Breitner
M Farkas-Dyck
winter
Yitzchak Gale