To: "Frank A. Christoph" <firstname.lastname@example.org>
Subject: Re: A propos de monad/About monads
From: John Prevost <email@example.com>
Date: 04 Oct 1999 14:55:33 -0400
In-Reply-To: "Frank A. Christoph"'s message of "Mon, 4 Oct 1999 17:40:10 +0900"
Frank A. Christoph <firstname.lastname@example.org> writes:
> But monads are also used in Opal, which is an eager language, to keep the
> base language pure from side-effects.
> Also, I often use monads in Haskell which have no side-effects at all. For
> example, I might use a non-imperative state transformer to "lay out the
> plumbing" for an algorithm, i.e., to avoid passing variables around
> explicitly; the error monad, which is a monad over what in Ocaml corresponds
> to the option type (functor), is also extremely useful, and has no
> side-effects either.
I spent some time last year working with monadic parsers--this is
another really nice way to use monads (especially if you have monad
comprehensions.) An example, using something like what I wrote to do
Monad comprehensions in camlp4:
let char c = << x | x <- item; x = c >>
let digit = << x | x <- item; x >= '0' && x <= '9' >>
let rec many p = << x::xs | x <- p; xs <- many p >>
let number = many digit
let bracket l p r = << x | _ <- l; x <- p; _ <- r >>
let rec seq p s = << x :: xs | x <- p; _ <- s; xs <- seq p s >>
|-| << [x] | x <- p >>
let number_list = bracket (char '[') (seq number (char ',')) (char ']')
And then number_list would parse something like "[1,2,3,4,56]" into
the Caml value [1; 2; 3; 4; 56].
Unfortunately, this kind of thing (along with other higher-order
combinator stuff for, as an example, formatted printing) doesn't work
that well in ML because of the value restriction. :(
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