Polymorphic variants question
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Date:   (:) 
From:  Olivier Andrieu <oandrieu@n...> 
Subject:  Re: [Camllist] Polymorphic variants question 
David Allsopp [Friday 1 September 2006] : > > Forgive the potentially obvious question  I'm not very familiar with > polymorphic variants but I think that they're what I want in this situation! > > Suppose I'm dealing with three constructors `A, `B and `C and I have a > function f that's supposed to take either `A or `C and return any of `A, `B > or `C. If I write: > > let f x = if x = `A then (true, `B) else (false, x) > > then I get the type > > val f : ([> `A  `B] as 'a) > bool * 'a > > Now, if I try to constrain it to what I'm after with > > let (f : [`A  `C] > bool * [`A  `B  `C]) = fun x > ... > > then I get a type error unless I change > (false, x) > to > (false, id x) > with > let id = function `A > `A  `C > `C > > Is there a better way of writing this? I'm using this in the context of > several interrelated lexers where `A, `B and `C are highlevel states and > certain lexers can only be called in a subset of those states but each lexer > may yield any value for the nextstate. I'd quite like to eliminate the id x > bit since it's only there to "separate" x from the return value for the > typechecker. you can use patternmatching with a "as" pattern to introduce another identifier for your value in the non`A branch : ,  # let f = function   `A > true, `B   `C as x > false, x ;;  val f : [< `A  `C ] > bool * [> `B  `C ] = <fun> ` (the [> `B  `C ] return type can be constrained to [`A`B`C] if you want)  Olivier