Re: [Sbcl-bugs] subtypep on function type

[Nikodemus S. <nik...@ra...>]

On 1 June 2012 04:20, Tomohiro Matsuyama <to...@cx...> wrote:

> I have found some odd subtypep behavior on function type.
>
> CL-USER> (subtypep '(function (fixnum) fixnum) '(function (fixnum) integer))
> T
> T
> CL-USER> (subtypep '(function (fixnum) fixnum) '(function (integer) integer))
> T
> T
> CL-USER> (subtypep '(function (fixnum) fixnum) '(function (bit) integer))
> NIL
> T
>
> As you may know, function parameters should be contravariant, meaning if we have a->b <: a'->b, then a' <: a must hold. So last two statement could be said wrong, or at least the second value should be false.

I don't follow. In CL, and SBCL specifically covariance seems exactly
right to me. Can you unpack your reasoning a little?

If I replace a (function (integer) integer) with (function (fixnum)
fixnum) any call-site that relied on the previous definition is still
safe: they may now get an error due to narrowing of the type, but any
type-based reasoning the system did is still valid. Ie. "if F returns,
the thing it returns must be an integer".

Of course CLHS SUBTYPEP doesn't specify SUBTYPEP properly -- so
perhaps we should return NIL, NIL in case of both covariance and
contravariance, and implement FSUBTYPEP (or possibly
FTYPES-COVARIANT-P and FTYPES-CONTRAVARIANT-P.)

Cheers,

 -- Nikodemus