Re: string/char vector type equivalence

On Mon, Feb 4, 2013 at 2:29 PM, Phil Clayton <[email protected]>wrote:

> On 04/02/13 03:36, Christopher Cramer wrote:
> > This compiles in MLton without error:
> >
> >       val foo: char vector = "foo"
> >       val bar: string = "bar"
> >       val baz: string = vector [#"b", #"a", #"z"]
> >       val _ = Vector.length foo
> >       val _ = size foo
> >       val _ = Vector.length bar
> >       val _ = size bar
> >       val _ = Vector.length baz
> >       val _ = size baz
> >       val _ = foo ^ bar ^ baz
> >
> > Not an earth-shattering discovery, but it does make MLton.CharVector
> > redundant...
>
> The FAQ (http://www.mlton.org/FAQ) even states:
>
>    Does MLton treat monomorphic arrays and vectors specially?
>
>    MLton implements monomorphic arrays and vectors (e.g. BoolArray,
>    Word8Vector) exactly as instantiations of their polymorphic
>    counterpart (e.g. bool array, Word8.word vector).
>
> so MLton.CharVector does seem superfluous.
>
>
> > The same sort of thing, with ints rather than chars, does not compile:
> >
> >       val foo: IntVector.vector = vector [1, 2, 3]
> >
> >       Error: intvector.sml 1.5.
> >         Pattern and expression disagree.
> >           pattern:    [IntVector.vector]
> >           expression: [int vector]
> >           in: (foo): IntVector.vector = vector [1, 2, 3]
> >       compilation aborted: parseAndElaborate reported errors
>
> This surprises me.  Given what is said in the FAQ, I would expect
> similar examples like
>
>    val a : BoolArray.array =
>      Array.fromList [true, false] : bool Array.array;
>
> to work.
>
> However, not allowing the above has the advantage that code will be more
> portable.


Agreed.  And, I would actually argue that the first example should have a
type error as well.  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.

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