[mercury-users] partial instantiation?

[Mark Brown]

On 19-Apr-2010, Tomas By <tomas at basun.net> wrote:
> On Mon, April 19, 2010 09:39, Ralph Becket wrote:
> > Tomas, Monday, 19 April 2010:
> >>
> >> Also, the way you have written it, the parameters are not passed to the
> >> commands.
> >
> > One of is mistaken here!
> 
> Well, my comment above was about your code, where the N was all alone by
> itself.
> 
> >  Given,
> >
> > :- pred cmd(string,list(cmdarg),cmdproc).
> > :- mode cmd(out,out,out) is nondet.
> >
> > cmd( "command", [],              dosomething(0,"filename") ).
> > cmd( "command", [str(F)],        dosomething(0,F)          ).
> > cmd( "command", [int(N)],        dosomething(N,"filename") ).
> > cmd( "command", [str(F),int(N)], dosomething(N,F)          ).
> >
> > if I have
> >
> > 	CmdNamd = "command",
> > 	F = "foo",
> > 	N = 42,
> > 	Args = [str(F), int(N)],
> > 	cmd(CmdNamd, Args, Command)
> >
> > then Command will be unified with dosomething(42, "foo").  Isn't
> > that what you need?
> 
> I thought we had established that the above will not compile,

I think Ralph meant

:- mode cmd(in, in, out) is semidet.

> but in the
> alternative universe where it works it is still not what I mean, because:
> 
> What I get on the command line is ["command","foo","42"],

So cmd is called with the (ground) value [str("foo"), int(42)] as the
second argument, and dosomething(42, "foo") is returned.

> but it could
> have been just ["command","42"]

In this case it is called with value [int(42)], and dosomething(42, "filename")
is returned.

> where "42" could have been a filename.
> 
> This is for experimentation. I want to be able to modify the commands
> and parameters easily. Doing a lot of fancy interpretation of the command
> line before I get to cmd/X seems misguided.

Thankfully Mercury does deep switch detection, so this simple implementation
is also compiled to efficient code.  :-)

Cheers,
Mark.

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