strictly matching monadic let and overloaded Bool (was: Are pattern guards obsolete?)
consider the following examples:
-- do-notation: explicit return; explicit guard; monadic result
d _ = do { Just b - return (Just True); guard b; return 42 }
-- list comprehension: explicit return; implicit guard; monadic (list) result
lc _ = [ 42 | Just b - return (Just True), b ]
-- pattern guard: implicit return; implicit guard; non-monadic result
pg _ | Just b - Just True, b = 42
This ongoing discussion has made me curious about whether we could actually
get rid of these irregularities in the language, without losing any of the
features
we like so much.
=== attempt 1
(a) boolean statements vs guards
this looks straightforward. Bool is a type, so can never be an instance of
constructor class Monad, so a boolean statement in a monadic context is
always invalid at the moment. that means we could simply extend our
syntactic sugar to take account of types, and read every
((e :: Bool) :: Monad m = m _)
in a statement of a do block as a shorthand for
(guard (e :: Bool) :: Monad m = m ())
(b) missing return in pattern guards
this could be made to fit the general pattern, if we had (return == id).
that would put us into the Identity monad, which seems fine at first,
since we only need return, bind, guard, and fail. unfortunately, those
are only the requirements for a single pattern guard - to handle not
just failure, but also fall-through, we also need mplus. which means
that the Identity monad does not have enough structure, we need at
least Maybe..
this first attempt leaves us with two problems. not only is (return==id)
not sufficient for (b), but the suggested approach to (a) is also not very
haskellish: instead of having syntactic sugar depend on type information,
the typical haskell approach is to have type-independent sugar that
introduces overloaded operations, such as
fromInteger :: Num a = Integer - a
to be resolved by the usual type class machinery. addressing these two
issues leads us to
=== attempt 2
(a) overloading Bool
following the approach of Num and overloaded numeric literals, we could
introduce a type class Boolean
class Boolean b where
fromBool :: Bool - b
instance Boolean Bool where
fromBool = id
and implicitly translate every literal expression of type Bool
True ~~ fromBool True
False ~~ fromBool False
now we can embed Boolean statements as monadic statements simply by
defining an additional instance
instance MonadPlus m = Boolean (m ()) where
fromBool = guard
(b) adding a strictly matching monadic let
we can't just have (return==id), and we do not want the hassle of having to
write
pattern - return expr
in pattern guards. the alternative of using let doesn't work either
let pattern = expr
because we do want pattern match failure to abort the pattern guard and
lead to overall match failure and fall-through. so what we really seem to want
is a shorthand notation for a strict variant of monadic let bindings. apfelmus
suggested to use '=' for this purpose, so that, wherever monadic generators
are permitted
pattern = expr ~~ pattern - return expr
===
returning to the examples, the approach of attempt 2 would allow us to write
-- do-notation: implicit return; implicit guard; monadic result
d _ = do { Just b = Just True; b; return 42 }
-- list comprehension: implicit return; implicit guard; monadic (list) result
lc _ = [ 42 | Just b = Just True, b ]
-- pattern guard: implicit return; implicit guard; non-monadic result
pg _ | Just b = Just True, b = 42
almost resolving the irregularities, and permitting uniform handling of related
syntactic constructs. hooray!-)
I say almost, because Bool permeates large parts of language and libraries,
so one would need to check every occurence of the type and possibly
replace Bool by (Boolean b = b). the Boolean Bool instance should mean
that this process could be incremental (ie, even without replacements, things
should still work, with more replacements generalizing more functionality,
similar to the Int vs Integer issue), but that hope ought to be tested in
practice.
one issue arising in practice is that we would like to have
fromBool :: MonadPlus m = Bool - m a
but the current definition of guard would fix the type to
fromBool :: MonadPlus m = Bool - m ()
which would require type annotations for Booleans used as guards. see the
attached example for an easy workaround.
on the positive side, this approach would not just make pattern guards more
regular, but '=' and 'MonadPlus m = Boolean (m ()) would be useful for
monadic code in general. even better than that, those of use doing embedded
DSLs in Haskell have been looking for a way to overload Bools for a long
time, and the implicit 'Boolean b = fromBool :: Bool - b' ought to get us
started in the right direction. most likely, we would need more Bool-based
constructs to be overloaded for
Re: Are pattern guards obsolete?
Donald Bruce Stewart [EMAIL PROTECTED] wrote: The joy of pattern guards reveals once you have more conditions. I wrote: Of course, this is not really the joy of pattern guards. It is the joy of monads, with perhaps a few character strokes saved by a confusing overloading of (-). Philippa Cowderoy wrote: I don't find it any more confusing than the overloading of -. You mean that it is used both for lambda abstractions and for functional dependencies? True, but those are so different that there is no confusion. Note that it's not (-) - it's not an operator. Right, it is syntactic sugar for a monad. But this syntax is already used in two places: do notation and list comprehensions. The semantics are exactly the same in both existing uses. The semantics of the proposed new use in pattern guards is quite different, as was discussed in the previous thread. Yet close enough to be confused. There seems to be a consensus that pattern guards are here to stay. So I am proposing to mitigate the damage somewhat by using a different but similar symbol . That matches the different but similar semantics. I mentioned (-) as one possibility. Regards, Yitz ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
On Wed, 13 Dec 2006, Yitzchak Gale wrote: Yitzchak Gale wrote: Of course, this is not really the joy of pattern guards. It is the joy of monads, with perhaps a few character strokes saved by a confusing overloading of (-). Philippa Cowderoy wrote: I don't find it any more confusing than the overloading of -. You mean that it is used both for lambda abstractions and for functional dependencies? True, but those are so different that there is no confusion. You missed out case statements. -- [EMAIL PROTECTED] My religion says so explains your beliefs. But it doesn't explain why I should hold them as well, let alone be restricted by them. ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
On Wed, 13 Dec 2006, Yitzchak Gale wrote: Philippa Cowderoy wrote: I don't find it any more confusing than the overloading of -. I wrote: You mean that it is used both for lambda abstractions and for functional dependencies? True, but those are so different that there is no confusion. Oh, and case. Also quite different. This is what I get for replying straight away! Anyway, existing problems are not an excuse to repeat the mistake and make matters even worse. I think my point is that I'm not aware of many people who actually think this is a problem or get confused. -- [EMAIL PROTECTED] There is no magic bullet. There are, however, plenty of bullets that magically home in on feet when not used in exactly the right circumstances. ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
Philippa Cowderoy wrote: This is what I get for replying straight away! Oh, no, I'm happy that you responded quickly. I think my point is that I'm not aware of many people who actually think this is a problem or get confused. Well, I don't mean that this is something that experienced Haskell programmers will stop and scratch their heads over. But the more of these kinds of inconsistencies you have, the worse it is for a programming language. The effect is cumulative. When there are too many of them, they make the language feel heavy, complex, and inelegant. They increase the number of careless errors. They put off beginners. Regards, Yitz ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
Hi, I am not clear why you think the current notation is confusing... Could you give a concrete example? I am thinking of something along the lines: based on how - works in list comprehensions and the do notation, I would expect that pattern guards do XXX but instead, they confusingly do YYY. I think that this will help us keep the discussion concrete. -Iavor On 12/13/06, Yitzchak Gale [EMAIL PROTECTED] wrote: Philippa Cowderoy wrote: This is what I get for replying straight away! Oh, no, I'm happy that you responded quickly. I think my point is that I'm not aware of many people who actually think this is a problem or get confused. Well, I don't mean that this is something that experienced Haskell programmers will stop and scratch their heads over. But the more of these kinds of inconsistencies you have, the worse it is for a programming language. The effect is cumulative. When there are too many of them, they make the language feel heavy, complex, and inelegant. They increase the number of careless errors. They put off beginners. Regards, Yitz ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
Iavor Diatchki wrote:
I am not clear why you think the current notation is confusing...
Could you give a concrete example? I am thinking of something along
the lines: based on how - works in list comprehensions and the do
notation, I would expect that pattern guards do XXX but instead, they
confusingly do YYY. I think that this will help us keep the
discussion concrete.
Pattern guards basically are a special-case syntactic sugar for
(instance MonadPlus Maybe). The guard
foo m x
| empty m = bar
| Just r - lookup x m, r == 'a' = foobar
directly translates to
foo m x = fromMaybe $
(do { guard (empty m); return bar;})
`mplus`
(do {Just r - return (lookup m x); guard (r == 'a');
return foobar;})
The point is that the pattern guard notation
Just r - lookup m x
does *not* translate to itself but to
Just r - return (lookup m x)
in the monad. The - in the pattern guard is a simple let binding. There
is no monadic action on the right hand side of - in the pattern guard.
Here, things get even more confused because (lookup m x) is itself a
Maybe type, so the best translation into (MonadPlus Maybe) actually would be
r - lookup m x
Regards,
apfelmus
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Re: Are pattern guards obsolete?
I am not clear why you think the current notation is confusing...
Could you give a concrete example? I am thinking of something along
the lines: based on how - works in list comprehensions and the do
notation, I would expect that pattern guards do XXX but instead, they
confusingly do YYY. I think that this will help us keep the
discussion concrete.
consider the following examples:
-- do-notation: explicit return; explicit guard; monadic result
d _ = do { Just b - return (Just True); guard b; return 42 }
-- list comprehension: explicit return; implicit guard; monadic (list) result
lc _ = [ 42 | Just b - return (Just True), b ]
-- pattern guard: implicit return; implicit guard; non-monadic result
pg _ | Just b - Just True, b = 42
in spite of their similarity, all of these constructs handle some of the
monadic aspects differently. the translations of pattern guards not only
embed statements in guard, they also embed the right hand sides of
generators in return. translations of list comprehensions only lift
statements. translation of do-notation lifts neither statements nor
generators.
does this clarify things?
Claus
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Re: Are pattern guards obsolete?
Hi in spite of their similarity, all of these constructs handle some of the monadic aspects differently. the translations of pattern guards not only embed statements in guard, they also embed the right hand sides of generators in return. translations of list comprehensions only lift statements. translation of do-notation lifts neither statements nor generators. does this clarify things? No. Pattern guards are obvious, they could only work in one particular way, and they do work that way. They make common things easier, and increase abstraction. If your only argument against them requires category theory, then I'd say that's a pretty solid reason for them going in. The argument that people seem to be making is that they are confusing, I completely disagree. f value | Just match - lookup value list = g match Without thinking too hard, I am curious how anyone could get the meaning of this wrong if they understand the rest of Haskell. Can you show a concrete example, where you think a user would get confused? Thanks Neil ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
Yitzchak Gale writes: Philippa Cowderoy wrote: I don't find it any more confusing than the overloading of -. I wrote: You mean that it is used both for lambda abstractions and for functional dependencies? True, but those are so different that there is no confusion. Oh, and case. Also quite different. Also type and kind signatures. The use in case and lambda abstractions strike me as analogous. They both have a pattern to the left and an expression to the right. -- David Menendez [EMAIL PROTECTED] | In this house, we obey the laws http://www.eyrie.org/~zednenem |of thermodynamics! ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
Donald Bruce Stewart [EMAIL PROTECTED] writes: The joy of pattern guards reveals once you have more conditions. Of course, this is not really the joy of pattern guards. It is the joy of monads, with perhaps a few character strokes saved by a confusing overloading of (-). But some people do seem to be used to this notation by now. So perhaps a good compromise would be to use a different operator for pattern guards, e.g. (-), instead of (-). What do you say? Yitz ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
Re: Are pattern guards obsolete?
On Wed, 13 Dec 2006, Yitz Gale wrote: Donald Bruce Stewart [EMAIL PROTECTED] writes: The joy of pattern guards reveals once you have more conditions. Of course, this is not really the joy of pattern guards. It is the joy of monads, with perhaps a few character strokes saved by a confusing overloading of (-). I don't find it any more confusing than the overloading of -. Note that it's not (-) - it's not an operator. -- [EMAIL PROTECTED] Sometimes you gotta fight fire with fire. Most of the time you just get burnt worse though. ___ Haskell-prime mailing list Haskell-prime@haskell.org http://www.haskell.org/mailman/listinfo/haskell-prime
