From: Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>
To: yminsky@cs.cornell.edu, yminsky@gmail.com
Cc: caml-list@inria.fr
Subject: Re: [Caml-list] When is a function polymorphic?
Date: Thu, 31 Mar 2005 11:42:53 +0900 (JST) [thread overview]
Message-ID: <20050331.114253.48852731.garrigue@math.nagoya-u.ac.jp> (raw)
In-Reply-To: <891bd339050330165142478f37@mail.gmail.com>
From: Yaron Minsky <yminsky@gmail.com>
> I think I screwed up the original examples a bit. I think the effect
> I'm looking at doesn't depend on the "as" notation in particular.
> Here's another example that doesn't use "as":
>
> # function Some x -> Some () | None -> None;;
> - : 'a option -> unit option = <fun>
> # function Some x -> Some () | x -> x;;
> - : unit option -> unit option = <fun>
>
> The reason I want this is for the following example. Consider some
> complicated union type with a single parameter:
>
> type 'a foo = A of 'a | B of int | C of string * string | ... | ZZ of float
>
> I want a function that converts an 'a foo to a unit foo. I tried to
> write it this way:
>
> function A _ -> A () | x -> x
>
> But this ends up having type: unit foo -> unit foo, which isn't what I
> want at all. Any idea of how to achieve this cleanly?
The effect you are looking for does depend on the "as" notation.
Here is how to write it:
function A _ -> A () | (B _ | C _ | ... | ZZ _) as x -> x
Note that the exhaustivity checker will correctly detect that you
second case is complete, and the pattern-matching compiler will just
branch on wheter the tag is A or not. I.e. no code will be produced to
check the second pattern.
As others pointed, polymorphic variants allow you to write it in a
shorter way, because you can define a type abbreviation for the
remaining cases, but in the end it uses the same "as" mechanism.
Jacques Garrigue
next prev parent reply other threads:[~2005-03-31 2:43 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2005-03-30 22:31 Yaron Minsky
2005-03-31 0:37 ` [Caml-list] " Jacques Garrigue
2005-03-31 0:51 ` Yaron Minsky
2005-03-31 1:15 ` Eric Cooper
2005-03-31 1:26 ` Martin Jambon
2005-03-31 2:42 ` Jacques Garrigue [this message]
2005-03-31 4:04 ` Yaron Minsky
2005-03-31 8:32 ` Jacques Garrigue
2005-03-31 12:04 ` Yaron Minsky
2005-03-31 12:48 ` Fermin Reig
2005-03-31 12:16 ` Jon Harrop
2005-04-01 16:26 ` Luc Maranget
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