Re: [Haskell-cafe] Lambda-case / lambda-if
On 10/2/10, Christopher Done chrisd...@googlemail.com wrote: On 2 October 2010 20:23, Max Bolingbroke batterseapo...@hotmail.com wrote: Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Nice work! I like it and have wanted it for a while, and I know many in the #haskell IRC channel would like it. The case is especially useful. Maybe the if is only useful sometimes. +1 for `case of'... I have called for it on many an occasion in #haskell. Thanks for implementing it, Max! I primarily see it as a way to remove a point from `\x - case x of ...', not a way to augment lambdas with pattern matching, like in `\0 - 1 \n k - k-1'. I suppose both of them work with monadic casing, with the `m =' trick. `\case' or `\case of' would work okay as well, the former Simon suggested, but some keywords *somewhere* would be nice, to avoid a perl-like procession of punctuation. (But the code golfer in me says otherwise.) Matt ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
To repeat, the analogues in SML and Erlang *do* support multiple clauses (as well as pattern matching) and the failure of Haskell lambdas to do so has always seemed like a weird restriction in a language that's usually free of weird restrictions. I agree with this sentiment. I have never understood why lambdas can't handle multiple clauses. I'd prefer to see something like \ 1 - f | 2 - g but I'm sure something could be worked out. This sounds sensible, since the lambda-case clause should really be about having a lambda with support for cases, not a case statement that's implicitly surrounded by a lambda. While I think the case of is a good idea, multiple clauses in lambdas seems more canonical to me. Nick ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 05/10/10 07:52, Nicolas Wu wrote:
I'd prefer to see something like
\ 1 - f
| 2 - g
but I'm sure something could be worked out.
While I think the case of
is a good idea, multiple clauses in lambdas seems more canonical to
me.
Alternatively, we could abandon lambdas and just use lambda-case.
All expressions like \1 - f become case of 1 - f
Multi-argument functions are a bit more verbose, as we effectively go
back to single argument functions with manual currying:
\x (C y) - z becomes: case of {x - case of {C y - z}}
There is the small matter of losing backwards compatibility, of course.
But on the plus side, this would reduce the number of constructions in
the language by one. (I think the strictness semantics, etc match up
for this transformation?).
;-)
Thanks,
Neil.
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Re: Re[2]: [Haskell-cafe] Lambda-case / lambda-if
On Tue, 5 Oct 2010 03:36:12 -0600, Luke Palmer lrpal...@gmail.com wrote: On Mon, Oct 4, 2010 at 9:04 PM, Dean Herington heringtonla...@mindspring.com wrote: With respect to datatype destructing functions, the Prelude has: maybe :: b - (a - b) - Maybe a - b either :: (a - c) - (b - c) - Either a b - c which suggests the following signatures for the analogues for Bool and list types: bool :: a - a - Bool - a list :: b - (a - [a] - b) - [a] - b This suggestion is not so clear to me. Maybe and Either are both non-recursive, so the Church and Scott encodings coincide. You've written the Scott encoding of list. The Church encoding should look familiar: list :: b - (a - b - b) - [a] - b I would argue for the previous one (Scott), since we already have this one (this is foldr with another order for arguments). -- Nicolas Pouillard http://nicolaspouillard.fr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
I also vote +1 for lambda-case, and abstain for lambda-if. I don't think multiple-clause lambdas being desirable should be an argument against lambda-case. After all, we can also define top-level functions with either multiple clauses or a single case expression. Haskell has always followed the TMTOWTDI school of thought with regards to syntax, as far as I know. And lambda-case has the notable advantage that someone has gone and implemented it. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
For what it's worth, after all this discussion my rather cheeky preference is as follows: Instead of introducing more specialised syntax, remove both existing special syntaxes for if and case, and introduce multi-clause support for lambdas! Cheers! On 2 October 2010 19:23, Max Bolingbroke batterseapo...@hotmail.com wrote: Hi Cafe, I implemented the proposed Haskell' feature lambda-case/lambda-if [1] during the Haskell Implementors Workshop yesterday for a bit of fun. The patches are online [2, 3]. The feature is demonstrated in this GHCi session: $ inplace/bin/ghc-stage2 --interactive -XLambdaCase GHCi, version 7.1.20101002: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude (if then Haskell else Cafe) False Cafe Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Max [1] http://hackage.haskell.org/trac/haskell-prime/ticket/41 [2] http://www.omega-prime.co.uk/files/LambdaCase-Testsuite.patch [3] http://www.omega-prime.co.uk/files/LambdaCase-Compiler.patch ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Ozgur Akgun ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
Richard O'Keefe schrieb:
I'd prefer to see something like
\ 1 - f
| 2 - g
but I'm sure something could be worked out.
In order to be consistent with current case, maybe in layout mode:
\1 - f
2 - g
and in non-layout mode
\{1 - f; 2 - g}
?
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Re: [Haskell-cafe] Lambda-case / lambda-if
-BEGIN PGP SIGNED MESSAGE-
Hash: SHA1
On 10/5/10 12:38 , Henning Thielemann wrote:
In order to be consistent with current case, maybe in layout mode:
\1 - f
2 - g
and in non-layout mode
\{1 - f; 2 - g}
+1; likewise for consistency it should support guards (which would preclude
using | the way Richard suggested, and goes along with the lambda-case thing).
- --
brandon s. allbery [linux,solaris,freebsd,perl] allb...@kf8nh.com
system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu
electrical and computer engineering, carnegie mellon university KF8NH
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Re: [Haskell-cafe] Lambda-case / lambda-if
Donn Cave d...@avvanta.com writes:
I think you're not the first to ask. Just out of curiosity, or is
there a use for these variations?
Just that they seem to be natural generalizations. If it's just the
single form of paramtrizing the condition, I think it's better served by
a regular function, 'bool' or (??) or whatever.
The reason for the initially proposed construct seems clear enough
to me, it's very much like `case'.
getargs = if then beTrue else beFalse . (==) [-t]
Isn't this equivalent, and only slightly more cumbersome?
getArgs = case of {True - beTrue; False - beFalse} . (==) [-t]
(And of course,
getArgs = case of [-t] - beTrue; _ - beFalse
is probably clearer anyway.)
-k
--
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Re: [Haskell-cafe] Lambda-case / lambda-if
Quoth Ketil Malde ke...@malde.org, ... Just that they seem to be natural generalizations. If it's just the single form of paramtrizing the condition, I think it's better served by a regular function, 'bool' or (??) or whatever. Well, yes, there's some logic to that. Like, bool b c a = if a then b else c getArgs = bool (putStrLn long) (putStrLn short) . ( 0) . length And I agree that's competitive with lambda-if as I understand it - though possibly not for the same reasons. For me, Haskell is not Lisp. Haskell's syntax takes a different direction, a mix of S-expression with stuff like if-then-else, and it works. If the lambda-if feature is actually useful in a way that takes advantage of the strength of the if-then-else notation, then I'm all for it. The problem is that due to the rarity of True/False as ... terminal value of a computation (I just made that up!), neither of these constructs is going to be worth much. Forget about lambda-if, even the regular function looks like hell - bool (putStrLn long) (putStrLn short) . ( 0) . length Compared to \ t - if length t 0 then putStrLn long else putStrLn short ... and much more so, with less trivial examples. In a brief survey of my own very small code base, I see only hIsEOF as a place where I could really use lambda-if. There, it would be vastly better than a regular bool function, but that's a pretty minimal use case. Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 5 October 2010 17:38, Henning Thielemann
schlepp...@henning-thielemann.de wrote:
Richard O'Keefe schrieb:
I'd prefer to see something like
\ 1 - f
| 2 - g
but I'm sure something could be worked out.
In order to be consistent with current case, maybe in layout mode:
\1 - f
2 - g
and in non-layout mode
\{1 - f; 2 - g}
Duncan Coutts also suggested this possibility to me - once I saw it
actually liked it rather better than the lambda-case stuff,
particularly since it generalises nicely to multiple arguments. I may
try to write a patch for this extension instead when I get some free
time.
To those asking where lambda-if comes from: it was just something I
hacked in while I was there - I don't have a particular motivation
example. It just seemed like a natural extension of the lambda-case
idea.
Cheers,
Max
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Re: [Haskell-cafe] Lambda-case / lambda-if
On 4 October 2010 00:38, Conal Elliott co...@conal.net wrote: I like it! Are the other sections available as well, e.g., (if False then else Cafe) Haskell -- Cafe They are not, though this would certainly make sense for lambda-if. It's not so clear with lambda-case because of the issue of free variables. Potentially we could support something like this, but it's a bit scary-looking: (case x of Just - ; Nothing -) (\y - I'm a Just) I'm a nothing Cheers, Max ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 6/10/2010, at 5:56 AM, Brandon S Allbery KF8NH wrote:
In order to be consistent with current case, maybe in layout mode:
\1 - f
2 - g
and in non-layout mode
\{1 - f; 2 - g}
+1; likewise for consistency it should support guards (which would preclude
using | the way Richard suggested, and goes along with the lambda-case
thing).
It's not that I particularly wanted |.
Just that I thought some sort of visual mark would be a good idea.
Forget I ever suggested it.
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Re: [Haskell-cafe] Lambda-case / lambda-if
-1 for if then. The examples of curried if then else look, to my
eyes, less readable than the pointed version. And it's easy enough to
write a 'bool' deconstructor, or an 'ifM' for the monadic case.
+1 for something to solve the dummy - m; case dummy of problem.
Here are the possibilities I can think of:
1) case of:
m = case of
Just _ - z | guard - a
_ - b
2) habit's case-
case- m of
Just _ - z | guard - a
_ - b
3) extended lambda (not sure what this would look like... would the
below parse with the give layout?)
m = \
Just _ - z | guard - a
_ - b
To me, #3 looks less ad-hoc and I like the idea of loosening a
restriction instead of introducing more sugar, but I'm not sure how
the syntax would work out. Also, from another point of view, 'f x =
...' is sugared to combine a \ and a case, while \ is unsugared, so
tacking some case sugar on to \ would introduce sugar in a previously
sugar-free area. Of course that \ is already sugared to curry
automatically, but if you rephrase this as add more sugar rather
than loosen a restriction it suddenly becomes less attractive since
now it's just sugar vs. sugar :)
#2 looks the nicest for this specific use, but seems less general than
#1. For instance, #1 allows f = case of { ... } . g.
So I like #1.
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Re: Re[2]: [Haskell-cafe] Lambda-case / lambda-if
At 3:36 AM -0600 10/5/10, Luke Palmer wrote: On Mon, Oct 4, 2010 at 9:04 PM, Dean Herington heringtonla...@mindspring.com wrote: With respect to datatype destructing functions, the Prelude has: maybe :: b - (a - b) - Maybe a - b either :: (a - c) - (b - c) - Either a b - c which suggests the following signatures for the analogues for Bool and list types: bool :: a - a - Bool - a list :: b - (a - [a] - b) - [a] - b This suggestion is not so clear to me. Maybe and Either are both non-recursive, so the Church and Scott encodings coincide. You've written the Scott encoding of list. The Church encoding should look familiar: list :: b - (a - b - b) - [a] - b Intuitively, a Scott encoding peels off one layer of datatype, whereas a Church encoding flattens down a whole recursive structure. Church encodings are more powerful -- you can do more without requiring a fixed point operator. Just to be clear, I am not arguing anything other than maybe and either don't readily generalize to list because of list's recursiveness. Luke Thanks, Luke, for pointing out the Church vs. Scott encoding issue. I agree with your conclusion (and feel better about the lack of the version of list I had suggested). Dean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
Max Bolingbroke batterseapo...@hotmail.com writes: [1] http://hackage.haskell.org/trac/haskell-prime/ticket/41 I tried to find anything about lambda-if in there, but failed (Trac and I aren't on very friendly terms, so it's probably my fault). Is there more information about the rationale and use cases for this? Prelude (if then Haskell else Cafe) False Cafe Presumably, this extends to Prelude (if False then else Cafe) Haskell Cafe and Prelude (if then Haskell else) False Cafe Cafe as well? My gut reaction is that this doesn't buy a whole lot, and that it is verbose and not very readable. Any examples where this is a win? Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q case of looks a bit weird, but I like the points brought up about avoiding to name a one-use variable (e.g., getArgs = case of ...) AFACS, this isn't easily implemented in Haskell either. -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Lambda-case / lambda-if
Hello Ketil, Monday, October 4, 2010, 11:30:48 AM, you wrote: Prelude (if then Haskell else Cafe) False lambda-if is easily implemented in terms of usual functions. and we even have one named bool: bool: Bool - a - a - a lambda-case cannot be implemented as a function since we need matching ability of case -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Lambda-case / lambda-if
On 4 October 2010 10:55, Bulat Ziganshin bulat.zigans...@gmail.com wrote: Hello Ketil, Monday, October 4, 2010, 11:30:48 AM, you wrote: Prelude (if then Haskell else Cafe) False lambda-if is easily implemented in terms of usual functions. and we even have one named bool: bool: Bool - a - a - a I agree, in fact I have bool here: http://hackage.haskell.org/packages/archive/higherorder/0.0/doc/html/Data-Bool-Higher.html And the corresponding other types: bool :: (a - b) - (a - b) - (a - Bool) - a - b list :: b - ([a] - b) - [a] - b maybe :: b - (a - b) - Maybe a - b But the case is especially useful for pattern matching. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
Quoth Ketil Malde ke...@malde.org, Max Bolingbroke batterseapo...@hotmail.com writes: ... Prelude (if then Haskell else Cafe) False Cafe Presumably, this extends to Prelude (if False then else Cafe) Haskell Cafe and Prelude (if then Haskell else) False Cafe Cafe as well? I think you're not the first to ask. Just out of curiosity, or is there a use for these variations? The reason for the initially proposed construct seems clear enough to me, it's very much like `case'. The difference is that of course it's limited to True False, so would naturally be used more with more `composition', e.g. getargs = if then beTrue else beFalse . (==) [-t] ... and thus will quickly become unreadable with less trivial components. Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
I really like the lambda-case. There are dozens of places in my code where I could use it. Not so sure about the lambda-if. It is just as easily done using an ordinary function. lambda-case: +1 lambda-if: neutral ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 4/10/2010, at 8:30 PM, Ketil Malde wrote: Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q case of looks a bit weird, but I like the points brought up about avoiding to name a one-use variable (e.g., getArgs = case of ...) AFACS, this isn't easily implemented in Haskell either. Erlang manages fine with multiclause 'fun': (fun (1) - One ; (_) - Not-one end)(1) ML manages fine with multiclause 'fn': (fn 1 = one | _ = not-one)(1) In both cases, the same notation is used for multiclause lambda as for single clause lambda. It seems excessively ugly to use completely different notation depending on the number of alternatives, especially when one of the notations has another, much more common, and distinct usage. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
Quoth Richard O'Keefe o...@cs.otago.ac.nz, ... Erlang manages fine with multiclause 'fun': (fun (1) - One ; (_) - Not-one end)(1) ML manages fine with multiclause 'fn': (fn 1 = one | _ = not-one)(1) In both cases, the same notation is used for multiclause lambda as for single clause lambda. It seems excessively ugly to use completely different notation depending on the number of alternatives, especially when one of the notations has another, much more common, and distinct usage. Just to be sure, are you saying, rather than case of 1 - f 2 - g you'd like to see \ support pattern matching etc. like named functions - \ 1 - f 2 - g ? Donn ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 5/10/2010, at 12:49 PM, Donn Cave wrote: Just to be sure, are you saying, rather than case of 1 - f 2 - g you'd like to see \ support pattern matching etc. like named functions - \ 1 - f 2 - g Absolutely.For the record, lambda DOES support pattern matching (Haskell 2010 report, section 3.3 lexp - \ apat1 ... apatn - exp Lambda abstractions are written \ p1 ... pn - e where the pi are _patterns_. ) To repeat, the analogues in SML and Erlang *do* support multiple clauses (as well as pattern matching) and the failure of Haskell lambdas to do so has always seemed like a weird restriction in a language that's usually free of weird restrictions. 'case of' is terminally cute. I dare say its inventor felt extremely proud of hacking it in, but it's the kind of thing that will have admirers swearing in frustration when they get tripped up by it yet again, and detractors sniggering. I'd prefer to see something like \ 1 - f | 2 - g but I'm sure something could be worked out. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Re[2]: [Haskell-cafe] Lambda-case / lambda-if
At 12:05 PM +0200 10/4/10, Christopher Done wrote: On 4 October 2010 10:55, Bulat Ziganshin bulat.zigans...@gmail.com wrote: Hello Ketil, Monday, October 4, 2010, 11:30:48 AM, you wrote: Prelude (if then Haskell else Cafe) False lambda-if is easily implemented in terms of usual functions. and we even have one named bool: bool: Bool - a - a - a I agree, in fact I have bool here: http://hackage.haskell.org/packages/archive/higherorder/0.0/doc/html/Data-Bool-Higher.html And the corresponding other types: bool :: (a - b) - (a - b) - (a - Bool) - a - b list :: b - ([a] - b) - [a] - b maybe :: b - (a - b) - Maybe a - b But the case is especially useful for pattern matching. I agree with others that lambda-if is better provided as a normal function rather than special syntax, that lambda-case is much more useful, and that it would be best if lambda-case were simply a generalization of anonymous lambdas (\ ...). With respect to datatype destructing functions, the Prelude has: maybe :: b - (a - b) - Maybe a - b either :: (a - c) - (b - c) - Either a b - c which suggests the following signatures for the analogues for Bool and list types: bool :: a - a - Bool - a list :: b - (a - [a] - b) - [a] - b (However, I do rather like the name (??) from Data.Bool.Higher, which is used for bool immediately above. And I would hesitate to use the name list for list destruction rather than construction. So I'm not about to propose adding these two functions.) Dean ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
I like it! Are the other sections available as well, e.g., (if False then else Cafe) Haskell -- Cafe - Conal On Sat, Oct 2, 2010 at 11:23 AM, Max Bolingbroke batterseapo...@hotmail.com wrote: Hi Cafe, I implemented the proposed Haskell' feature lambda-case/lambda-if [1] during the Haskell Implementors Workshop yesterday for a bit of fun. The patches are online [2, 3]. The feature is demonstrated in this GHCi session: $ inplace/bin/ghc-stage2 --interactive -XLambdaCase GHCi, version 7.1.20101002: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude (if then Haskell else Cafe) False Cafe Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Max [1] http://hackage.haskell.org/trac/haskell-prime/ticket/41 [2] http://www.omega-prime.co.uk/files/LambdaCase-Testsuite.patch [3] http://www.omega-prime.co.uk/files/LambdaCase-Compiler.patch ___ 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
[Haskell-cafe] Lambda-case / lambda-if
Hi Cafe, I implemented the proposed Haskell' feature lambda-case/lambda-if [1] during the Haskell Implementors Workshop yesterday for a bit of fun. The patches are online [2, 3]. The feature is demonstrated in this GHCi session: $ inplace/bin/ghc-stage2 --interactive -XLambdaCase GHCi, version 7.1.20101002: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude (if then Haskell else Cafe) False Cafe Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Max [1] http://hackage.haskell.org/trac/haskell-prime/ticket/41 [2] http://www.omega-prime.co.uk/files/LambdaCase-Testsuite.patch [3] http://www.omega-prime.co.uk/files/LambdaCase-Compiler.patch ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
Max == Max Bolingbroke batterseapo...@hotmail.com writes: Prelude (if then Haskell else Cafe) False Max Cafe My reaction is to ask: Can you write this as: (if then else) False Haskell Cafe ? -- Colin Adams Preston Lancashire () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On Sat, 2 Oct 2010, Max Bolingbroke wrote: Hi Cafe, I implemented the proposed Haskell' feature lambda-case/lambda-if [1] during the Haskell Implementors Workshop yesterday for a bit of fun. The patches are online [2, 3]. The feature is demonstrated in this GHCi session: $ inplace/bin/ghc-stage2 --interactive -XLambdaCase GHCi, version 7.1.20101002: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude (if then Haskell else Cafe) False Cafe Prelude (case of 1 - One; _ - Not-one) 1 One Prelude :q Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Nice! Concerning if-then-else I would more like to see an according function to go to Data.Bool, then we won't need more syntactic sugar like if-then-else. However the lambda-case would be useful for me. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On Sat, 2 Oct 2010, Colin Paul Adams wrote: Max == Max Bolingbroke batterseapo...@hotmail.com writes: Prelude (if then Haskell else Cafe) False Max Cafe My reaction is to ask: Can you write this as: (if then else) False Haskell Cafe ? Sure: ifThenElse :: Bool - a - a - a ifThenElse True x _ = x ifThenElse False _ y = y Prelude ifThenElse False Haskell Cafe (I have done this in utility-ht, and called it if'.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 2 October 2010 20:23, Max Bolingbroke batterseapo...@hotmail.com wrote: Do you like this feature and think it would be worth incorporating this into GHC? Or is it too specialised to be of use? If there is enough support, I'll create a ticket and see what GHC HQ make of it. Nice work! I like it and have wanted it for a while, and I know many in the #haskell IRC channel would like it. The case is especially useful. Maybe the if is only useful sometimes. A benefit for lambda-case that I'll throw in the mix is: main = do args - getArgs case args of [path] - do exists - doesFileExist filepath if exists then readFile filepath = putStrLn else error file does not exist _ - error usage: foo filename becomes: main = do getArgs = case of [path] - doesFileExist filepath = if then readFile filepath = putStrLn else error file does not exist _ - error usage: foo filename There's nothing more annoying than having to introduce intermediate bindings when you're going to immediate pattern match against it immediately and never use it again. It's both annoying to have to think of a variable name that makes sense and is not in scope or will be in scope, and annoying to type it out, and it's just ugly. This is *not* a special-case, it happens all the time and it's one of the few things in the syntax I wish could be updated. I vote yes, yes, and double yes! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
I just had a look at hpaste.org, and, amusingly, the first paste has this: down - openLazyURI http://list.iblocklist.com/?list=bt_level1; case down of Left _ - error Could not download file Right bs - do input - bs ... I can collect a huge list of instances of this annoying pattern from Hackage and Google Code Search if it will encourage GHC HQ to make it an extension. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 2 October 2010 19:33, Henning Thielemann lemm...@henning-thielemann.dewrote: On Sat, 2 Oct 2010, Max Bolingbroke wrote: ... lambda-case/lambda-if ... Nice! Concerning if-then-else I would more like to see an according function to go to Data.Bool, then we won't need more syntactic sugar like if-then-else. However the lambda-case would be useful for me. And I was just reading this entry in the wiki: http://www.haskell.org/haskellwiki/If-then-else Best, Ozgur ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 02.10.2010, at 20:35, Henning Thielemann wrote: On Sat, 2 Oct 2010, Colin Paul Adams wrote: Prelude (if then Haskell else Cafe) False Max Cafe My reaction is to ask: Can you write this as: (if then else) False Haskell Cafe ? Sure: ifThenElse :: Bool - a - a - a ifThenElse True x _ = x ifThenElse False _ y = y Prelude ifThenElse False Haskell Cafe You can use a similar approach for case expressions ; ) import Prelude hiding ( catch ) import System.IO.Unsafe ( unsafePerformIO ) import Control.Exception ( catch, evaluate, PatternMatchFail ) caseOf :: a - [a - b] - b caseOf x = unsafePerformIO . firstMatch . map ($x) firstMatch :: [a] - IO a firstMatch (x:xs) = catch (evaluate x) (handlePatternFail (firstMatch xs)) handlePatternFail :: a - PatternMatchFail - a handlePatternFail x _ = x test = (flip caseOf [\1 - One, \_ - Not-one]) 1 Well, to tell the truth this does not work correctly as the following example shows. test2 = (flip caseOf [\1 - ((\2 - One) 3), \_ - Not-one]) 1 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On Sat, Oct 2, 2010 at 4:13 PM, Christopher Done chrisd...@googlemail.com wrote: There's nothing more annoying than having to introduce intermediate bindings when you're going to immediate pattern match against it immediately and never use it again. It's both annoying to have to think of a variable name that makes sense and is not in scope or will be in scope, and annoying to type it out, and it's just ugly. This is *not* a special-case, it happens all the time and it's one of the few things in the syntax I wish could be updated. I vote yes, yes, and double yes! I wholly agree with Christopher and for the same reason, +1. Thanks, -- Felipe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 On 10/2/10 15:27 , Jan Christiansen wrote: You can use a similar approach for case expressions ; ) There are several better (that is, not using unsafePerformIO) versions at http://haskell.org/haskellwiki/Case . - -- brandon s. allbery [linux,solaris,freebsd,perl] allb...@kf8nh.com system administrator [openafs,heimdal,too many hats] allb...@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH -BEGIN PGP SIGNATURE- Version: GnuPG v2.0.10 (Darwin) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iEYEARECAAYFAkynmNsACgkQIn7hlCsL25XbOgCfdjFrXdR3PWJvPUif7VVfZZak lOcAoMpp6l1+XOxU6vwCT+sgLI94l3Kx =+gFp -END PGP SIGNATURE- ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lambda-case / lambda-if
On 10/2/10 3:13 PM, Christopher Done wrote:
There's nothing more annoying than having to introduce intermediate
bindings when you're going to immediate pattern match against it
immediately and never use it again. It's both annoying to have to
think of a variable name that makes sense and is not in scope or will
be in scope, and annoying to type it out, and it's just ugly. This is
*not* a special-case, it happens all the time and it's one of the few
things in the syntax I wish could be updated.
+1.
In Mark Jones' new language, Habit, they have monadic versions of case
and if-then-else for precisely this reason.
I'm not sure if the (case of {...}) syntax is the best one to use for
this feature, but I'd love to get rid of those intermediate names for
monadic case expressions.
--
Live well,
~wren
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