Re: string/char vector type equivalence

On Wed, Feb 6, 2013 at 11:29 AM, Christopher Cramer <
[email protected]> wrote:

> On Tue, Feb 05, 2013 at 09:31:56AM -0500, Matthew Fluet wrote:
> > I think that the reason that it doesn't is to support
> > string constants, but I couldn't quickly see where the necessary type
> > equalities were established in the Basis Library implementation and the
> > type checker.
>
> Actually, with some further experimentation, it seems like the fix should
> be
> rather simple, as this does not compile:
>
>         structure Vector :> sig
>                 eqtype 'a vector
>                 val fromList: 'a list -> 'a vector
>         end = struct
>                 type 'a vector = 'a vector
>                 val fromList = vector
>         end
>
>         type 'a vector = 'a Vector.vector
>         val vector = Vector.fromList
>
>         val foo: char vector = "foo"
>         val bar: string = vector [#"b", #"a", #"r"]
>         val _ = foo ^ bar
>
>         Error: opaquevector.sml 12.5.
>           Pattern and expression disagree.
>             pattern:    [char vector]
>             expression: [string]
>             in: (foo): char vector = "foo"
>         Error: opaquevector.sml 13.5.
>           Pattern and expression disagree.
>             pattern:    [string]
>             expression: [char vector]
>             in: (bar): string = vector [#"b", #"a", #"r"]
>         Error: opaquevector.sml 14.13.
>           Function applied to incorrect argument.
>             expects: [string] * _
>             but got: [char vector] * _
>             in: ^ (foo, bar)
>
> Since there are no vector literals, it should be OK to do something
> like this in the Basis Library without touching the type checker.
>

That is a very nice observation.  I'll investigate that a little more,
although I seem to recall that we couldn't use as many opaque signature
matches as we wished, because of the various ways in which some types are
used in signatures.

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