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ANN: pattern guards
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Date: 2007-06-30 (04:44)
From: Arnaud Spiwack <aspiwack@l...>
Subject: Re: [Caml-list] ANN: pattern guards
A (too) quick answer could of course be: "in the absence of a 'with 
clause', it is considered the identity 'with clause'". But I'm not sure 
it's that satisfying in practice. (it precisely and accurately adresses 
this very case John Skaller is raising, but not the same example with an 
additional variable in each constructor...).

A suitable solution for nested guarded patterns could be, instead of 
saying "all the branches of the or-pattern must agree on the variable", 
to use a less coercive, but more compromising law : "only the variable 
common in all the branches of the or-pattern are bound in the match 
branch". This sentence uses "branch" far too many times, which makes it 
awkward and slighlty evil, but the principle is there :

| A x | B t -> %1

Would bind nothing in %1, but still be legal (should raise "unused 
variable" warnings though). More interestingly :

| C x with z=3 and w=2*x | D z r s v with w = r+s+v -> %2

Would bind z and w only in %2 (without "unused variable" warning this 
time, provided %2 contains z and w).

At the moment I'm writing this mail, it sounds like a safe way to 
proceed. And to be resemble what Jeremy Yallop was suggesting in his 
last mail as well.

PS : Slight variants this solution, which I would consider messier 
myself, would be to say that "only the with -clause pattern variables 
are considered bound in the branch body", as Skaller suggested. And that 
an identity with clause is added "each time it is necessary for the 
branch body" (though this is not clear in case of naming conflict) or 
"each time is is necessary for all the branches to have the same set of 
variables" (plus the "no with is all identities" thingy) which may be 
safe as well (but the original one sounds more flexible).

Arnaud Spiwack

skaller a écrit :
> On Fri, 2007-06-29 at 19:56 +0100, Jeremy Yallop wrote:
>> skaller wrote:
>>> On Fri, 2007-06-29 at 15:19 +0100, Jeremy Yallop wrote:
>>>> I'm pleased to announce the initial release of `patterns', an OCaml
>>>> extension providing general-purposes additions to pattern matching.
>>> I want to do this:
>>> 	match x with
>>> 	| Y x with a=x and b=x
>>> 	| X (y,z) with a=y and b=z
>>> 	-> f a b
>> Interesting.  Do you want 'z' to be in scope in the guards ("a=y" etc.) 
>> but not in the expression ("f a b")?  Or do you just generally want to 
>> allow or-patterns where the branches have different bindings as long as 
>> the expression only uses variables that are bound in every branch?
> Good question. I don't know. In theory, the idea is a 
> 'change of variables' as in a coordinate transformation, so only the
> 'final' variables should be in scope, i.e. 'z' would not be in scope.
> In practice, a suitable syntax needs to be devised which is convenient
> to use: a common case would be:
> 	| X with x = 1
> 	| Y x -> f x
> and it would be messy to have to write the identity change of variables
> in the second branch.. so I'm open to suggestions as to syntax.
>>> This won't work at the moment for two reasons:
>>> * I assume the precedence of 'with' is the same as 'when',
>>>   which is not convenient
>> Right: "with" scopes over an entire match-case, which might include 
>> or-patterns, just as with "when".
> Which is a pain, you can't write:
> 	(
> 	| X 
> 	| Y x when f x 
> 	| Z x when g x 
> 	)
> 	-> ....
> [Felix allows nested 'when' clauses but not alternatives .. the latter
> due to laziness on my part implementing it]
>>> * the variables in the basic patterns don't agree
>>> The whole point of the above is to switch all the branches
>>> to normalised variables. At the moment I have to write:
>>> 	match x with
>>> 	| Y x -> f x x
>>> 	| X (y,z) -> f y z
>> Unless I'm mistaken you can write this as
>>      match x with
>>      | Y (y as z)
>>      | X (y,z)    -> f y z
>> Is there some more general case for which this won't work out?
> Of course! See above. Conceptually you need an arbitrary
> change of variables. For example:
> 	| Polar (r, theta) with z = polar r theta
> 	| Cartesian (x,y)  with z = cartesian x y
> 	-> f z
> As far as I can see this is basically eta-expansion,
> known to dummies like me as a 'wrapper function',
> which for functions allows you call a function with one
> set of variables with a completely different set of variables
> by a standard change of variables
> The idea is basically that, but 'moved' to the other
> side of the -> sign in a pattern match. The above case
> can of course be written:
> 	| Polar (r, theta) -> let z = polar r theta in f z
> 	| Cartestian (x,y) -> let z = cartesian x y in f z
> but involves duplicating the call to f.
> BTW: I'm writing some basic Scheme at the moment and I'm struck
> by how much is lost, not having pattern matching -- yet
> of course it is almost all just sugar.
> BTW2: It also strikes me good syntactic design is a tradeoff
> between the tensions of avoiding duplication and gratuitous
> invention, retaining localisation (things should be 
> defined near where they're used), and modularity
> (name anything complex).
> So for example simple anonymous functions are good
> (localisation), let/in is good (factor complexity
> but retain localisation) and C++ sucks (loss of
> localisation).