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A propos de monad
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Date: | 1999-10-05 (08:12) |
From: | John Prevost <prevost@m...> |
Subject: | Re: A propos de monad/About monads |
Frank A. Christoph <christo@nextsolution.co.jp> 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. :( John.