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 09:37:24 +0900 (JST) [thread overview]
Message-ID: <20050331.093724.70528102.garrigue@math.nagoya-u.ac.jp> (raw)
In-Reply-To: <891bd33905033014311636570a@mail.gmail.com>
From: Yaron Minsky <yminsky@gmail.com>
> I don't understand the following results. It seems like these two
> examples should have the same type. In this example, there isn't much
> of a difference between the two cases, but there are cases where this
> idiom is quite convenient. Any idea how to salvage it?
>
> # function Some x -> Some () | None as x -> x;;
> - : 'a option -> unit option = <fun>
> # function Some x -> Some () | x -> x;;
> - : unit option -> unit option = <fun>
The typechecker does something special about "as x" patterns.
Namely, rather than unifying the type of x with the type of the whole
input, it types the aliased pattern twice, and only unifies type
parameters which appear in the pattern. So this means that
# function Some _ as x -> x | None -> Some () ;;
- : unit option -> unit option = <fun>
since the type parameter appears in the argument to Some, while
# function Some x -> Some () | None as x -> x;;
- : 'a option -> unit option = <fun>
since it doesn't appear in None.
This clever typing only happens with "<pat> as <var>" patterns, so
your second example gets the standard result (it is assumed that x
could be anything, including Some, as the typing does not use the
result of the exhaustiveness analysis.)
Jacques Garrigue
next prev parent reply other threads:[~2005-03-31 0:37 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 ` Jacques Garrigue [this message]
2005-03-31 0:51 ` [Caml-list] " Yaron Minsky
2005-03-31 1:15 ` Eric Cooper
2005-03-31 1:26 ` Martin Jambon
2005-03-31 2:42 ` Jacques Garrigue
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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