Subject: Re: Infinite Streams in Caml Light 0.5
To: hedden@eniac.seas.upenn.edu (Jerry Hedden)
Date: Wed, 28 Oct 92 10:23:43 MET
In-Reply-To: <9210271942.AA00591@eniac.seas.upenn.edu>; from "Jerry Hedden" at Oct 27, 92 2:42 pm
From: Michel.Mauny@inria.fr (Michel Mauny)
> There is a point concerning streams in Caml Light that is not
> dealt with in the manual. This involves the use of function
> calls in stream patterns. Such calls are not evaluated in a lazy
> manner, and can cause the system to crash when a recursive
> function is used on infinite streams. For example, the following
> function is intended to map a function over all the elements of a
> stream:
>
> let rec map_stream func = function
> [< 'x; (map_stream func) strm >] -> [< '(func x); strm >]
> | [< >] -> [< >]
> ;;
>
> However, as indicated above, if `strm' is infinite, evaluation of
> the recursive call progresses ad infinitum until "out of memory"
> occurs.
Yes, you're right: this fact should be clearly noticed in the manual.
However, this behavior of stream matching is not surprising: as
pattern-matching has a strict behavior in lazy languages,
stream-matching is also strict. One has to check completely the
left-hand part before giving control to the right-hand part of
matching rules. But I admit that it is easy to make such a mistake (I
do it quite often).
Michel Mauny