On Wed, 2007-04-11 at 19:58 -0700, Erick Tryzelaar wrote:
> A couple more, not necessarily having to do with the paper:
>
> 1. warn if not exhaustively matching all patterns.
>
>
> 2. aggregating patterns:
>
> match x with
> | 5 | 6 => ...
> | 7 => ...
> | _ => ...
> endmatch
>
> and with variable binding:
>
> match x with
> | Cons(?x, Cons(_, _)) | Cons(?x, Empty[int]) => ...
> | _ => ...
> endmatch
It needs:
| (X (?x,?y) | Y (1,?y) with x = 99) => ...
i.e. a way to synthesise variables. I proposed this for Ocaml too.
The match construction is ugly:
match e with
| X ?y => y+x
is really:
let fun f x => y + x in
match e with
| X ?x => f x
but there's no 'let fun' construction. With this representation:
| (X (?x,?y) | Y (1,?y) with x = 99) => ...
just becomes
let fun f (x,y) = .. in match . . with
| X (?x,?y) => f (x,y)
| Y (1,?y) => f (99,x)
Remember let ?a = b in c is just sugar for
match b with ?a => c
The problem here, trivially, is that there's no 'scope'
in which to put the match handler abstraction, because it's
a function. Of course you CAN do it by:
let f = (fun x,y) => ..) in match . . with
| X (?x,?y) => f (x,y)
| Y (1,?y) => f (99,x)
i.e. use a lambda expression.
Felix pattern match handlers automatically inline such
a notional handler: in fact what it does is something
like:
let e = match_argument in
if match_X e then f (x,y) replacing
x = extractor_x e
y = extractor_y e
else if match_Y e then f (99,x) replacing ..
i.e. it directly replaces x,y etc in the argument
term .. BEFORE type checking .. with extractors
for the variables.
The extractor for y in
(_,?y)
is just the projection called 'snd' in Ocaml. The match
checker is the identity (tuple patterns always match).
The extractor for a union member | X y with
pattern | X ?y is just
ctor_arg (u) ::= u.data (cast to arg type)
and the match checker is
X = index u ::= X = u.variant
where u is just a _uctor_.
What we REALLY need is Jay pattern matching, but that requires
actively building up the extractors at run time.
--
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net
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