From: Jeremy Yallop <jeremy.yallop@ed.ac.uk>
To: caml-list@inria.fr
Subject: ANN: lazy patterns (patterns v 0.3)
Date: Thu, 25 Oct 2007 22:37:40 +0100 [thread overview]
Message-ID: <47210CA4.60707@ed.ac.uk> (raw)
I'm pleased to announce a new release of `patterns', an OCaml
extension providing general-purposes additions to pattern matching.
This release includes a new feature, "lazy patterns".
You can download patterns from
http://code.google.com/p/ocaml-patterns/
Lazy patterns extend OCaml pattern syntax with the keyword "lazy",
mirroring the use of lazy in expressions. With this extension
patterns can be used to deconstruct lazy values; for example, you can
define a function that behaves like Lazy.force as follows
let force (lazy x) = x
or, given a type of lazy lists in the "odd style":
type 'a llist = Nil | Cons of 'a * lazy llist
you can write a lazy map function:
let rec map f = function
| Nil -> Nil
| Cons (h, lazy t) -> Cons (f h, lazy (map f t))
You can use "lazy" anywhere you can use a constructor: in nested
patterns, or-patterns, patterns for "let", "function", "match",
"try/with", etc. Lazy patterns can also be used with the other
extension provided in the current release, pattern guards: you can
write, for example,
match v with
| A (x, y) with lazy (z,w) = f x -> e
Paradoxically, lazy patterns make pattern-matching more eager, since
they force evaluation of delayed values. However, no more forcing
than necessary (for some suitable definition thereof) will occur: the
following will return "true", for example.
match lazy 3, lazy (assert false) with
| lazy 2, lazy x -> false
| lazy 3, _ -> true
Documentation for lazy patterns will be available soon. Comments are
welcome, bug reports especially so.
A caveat: due to the translation used, lazy patterns can sometimes
give rise to spurious warnings. For example, for the following
function
function
lazy (Some _) -> 1
| lazy None -> 0
OCaml gives the warning
"Warning X: bad style, all clauses in this pattern-matching are
guarded."
It is of course possible to avoid these warnings by using "-w x" or by
adding redundant match cases.
For information on pattern guards see the previous announcements
v0.1: http://groups.google.com/group/fa.caml/msg/b0ec5324180bfeba
v0.2: http://groups.google.com/group/fa.caml/msg/016e76d7a51559c8
and the documentation on the website
http://code.google.com/p/ocaml-patterns/wiki/PatternGuards
Jeremy.
reply other threads:[~2007-10-26 7:02 UTC|newest]
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