Pattern guards vs. case (was, unfortunately :Re: interesting example of laziness/ghc optimisation)
(Apologies, I forgot to change the subject) Laszlo Nemeth <laszlo@ropas.kaist.ac.kr> writes:
* * * Ketil Malde <ketil@ii.uib.no> wrote:
There is no difference. The 'pipe-syntax' (or pattern guards) gets desugared (by the pattern matching compiler) to case statements i.e.:
runRandom = \ last max num.case (num > 1) of True -> runRandom (fst new) max (num-1) False -> snd new
But - the converse is not true, is it? I can write ... = case foo of (Foo f) -> ... (Bar b) -> ... but I can't express that as a pattern-guarded expression, can I? My impression here is that the PG syntax adds nothing, and is hardly any more readable, and less intuitive for migrators from more traditional language. Why is it there at all? Is there a (rough) guideline for when to use one or the other?
For a more detailed discussion see SPJ's book, Augustsson original paper, or M Pettersen't thesis (LNCS 1549).
Okay, I'll try to look them up. -kzm -- If I haven't seen further, it is by standing in the footprints of giants
On Thu, Mar 01, 2001 at 09:40:48AM +0100, Ketil Malde wrote:
(Apologies, I forgot to change the subject)
Laszlo Nemeth <laszlo@ropas.kaist.ac.kr> writes:
* * * Ketil Malde <ketil@ii.uib.no> wrote:
There is no difference. The 'pipe-syntax' (or pattern guards) gets desugared (by the pattern matching compiler) to case statements i.e.:
runRandom = \ last max num.case (num > 1) of True -> runRandom (fst new) max (num-1) False -> snd new
But - the converse is not true, is it? I can write
... = case foo of (Foo f) -> ... (Bar b) -> ...
but I can't express that as a pattern-guarded expression, can I? My impression here is that the PG syntax adds nothing, and is hardly any more readable, and less intuitive for migrators from more traditional language. Why is it there at all? Is there a (rough) guideline for when to use one or the other?
this is because pattern guards are not the same as case statements, they do not do matching and deconstruction of values, but rather are boolean expressions to be checked _in order_ to determine whether to use that function body or not. imagine f n | n < 0 = -1 f n | n < 5 = 1 f n = 2 this cannot be expressed as a single case statement because you cannot match 'n' against an expression. (just like you cannot deconstruct a value in a boolean expression as your above example demonstrates) this translates to nested if statements rather than a single case statement. what is confusing is that if statements can be constructed from case statements too... the above translates to f n = if n < 0 then -1 else if n < 5 then 1 else 2 or in case notation case n < 0 of True -> -1 False -> case n < 5 of True -> 1 False -> 2 note you will need many nested case statements to deal with patter guards, they are not equivalent to a single case statement. in the core language 'case' is the ONLY alternation primitive so everything can translate to it at some point. pattern guards are useful because otherwise you end up with heavily nested if or case statements which are definately less readable than the pattern-guard syntax. hint: read '|' as 'such that' so f n | n < 0 = ... reads "f of n such that n is less than zero is ..." note also that the vertical bar interpreted in this way also has mathematical precident so its not completely arbitrary. Hope this helps and that I am not utterly wrong :) John -- -------------------------------------------------------------- John Meacham http://www.ugcs.caltech.edu/~john/ California Institute of Technology, Alum. john@foo.net --------------------------------------------------------------
* * * Ketil Malde <ketil@ii.uib.no> wrote:
ut - the converse is not true, is it? I can write
... = case foo of (Foo f) -> ... (Bar b) -> ...
ut I can't express that as a pattern-guarded expression, can I?
You probably have already seen John's reply which pretty much says everything. But if you want to be really weird you can write something like (I haven't typed this in): f x | (Foo _) == x = f x | (Bar _) == x = which doesn't buy you anything because it doesn't bind the argument of Foo to anything. There were a couple of messages a year or so ago about what can be done with pattern matching and I recall Simon Marlow posted a few puzzling examples.
Why is it there at all? Is there a (rough) guideline for when to use one or the other?
Sugar ... but I think in this case is not the 'unhealthy' variety. As John noted, without it you would have to write nested case statements which is cumbersome and may lead to suboptimal code i.e. code duplication and repeated tests (assuming later stages of your compiler doesn't rearrange the code). In fact, the entire issue of compilation of pattern matching arises from the desire to select the appropriate rhs with the minimum number of tests. Rough guideline? I use guards whenever I can (which means not at all these days since SML sadly lacks guards (for a good reason!)), because I find it easy to read it and I hope that a clever pattern matching compiler takes advantage of the extra information. --laszlo
But if you want to be really weird you can write something like (I haven't typed this in):
f x | (Foo _) == x = f x | (Bar _) == x =
no, you can't, e. g. hugs (Feb 2000) says Prelude> let f x | Just 4 == x = 0 in f (Just 4) 0 Prelude> let f x | Just _ == x = 0 in f (Just 4) ERROR: Illegal `_' in expression -- -- Johannes Waldmann ---- http://www.informatik.uni-leipzig.de/~joe/ -- -- joe@informatik.uni-leipzig.de -- phone/fax (+49) 341 9732 204/252 --
* * * Johannes Waldmann wrote:
But if you want to be really weird you can write something like (I haven't typed this in):
f x | (Foo _) == x = f x | (Bar _) == x =
no, you can't, e. g. hugs (Feb 2000) says
Prelude> let f x | Just 4 == x = 0 in f (Just 4) 0 Prelude> let f x | Just _ == x = 0 in f (Just 4) ERROR: Illegal `_' in expression
So it works with (Just 4), but it doesn't with (Just _)? I can't see anything in the syntax (Haskell report pg 127, 128) which rules the second one out. Though, I'd try it with parenthesis. Shall we call it a bug (unless I am missing something obvious)? --laszlo
Thu, 1 Mar 2001 20:25:28 +0900 (KST), Laszlo Nemeth <laszlo@ropas.kaist.ac.kr> pisze:
Prelude> let f x | Just _ == x = 0 in f (Just 4) ERROR: Illegal `_' in expression
So it works with (Just 4), but it doesn't with (Just _)?
'Just _ == x' must be an exppression. '_' is a pattern but not an expression. -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK
participants (5)
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Johannes Waldmann -
John Meacham -
Ketil Malde -
Laszlo Nemeth -
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