[nem-en] Pattern matching using `is' loses bindings

[Snaury]

On 7/27/06, Kamil Skalski <kamil.skalski at gmail.com> wrote:
> In your example I would just use:
> >   def x = 18 /* or 35, 8, etc */;
> >   match((x, foo(x)) {
> >     | (x, Some (a,b)) when x > 10 =>
> >       System.Console.WriteLine($"x > 10 leads to ($a,$b)");
> >     | (x, Some (a,b)) when x > 5 =>
> >       System.Console.WriteLine($"x < 5 leads to ($a,$b)");
> >     | (x, _) =>
> >       System.Console.WriteLine($"x = $x");
> >   }

Actually, I had different calls there: for one case it was foo() and
for the other it was fee(). And that wasn't really a real world
example, it was just to show difficulties. Doing (x, foo(x)) won't
help, because sometimes, depending on argument it might need to call
different functions (foo(x) in that case are functions to
check-and-transform x, and I imagined to use match to route different
x to different transform functions, which might fail and then try next
one), and in some cases it might even be meaningless to call any
foo(), plus different foo() might return different results. Anyway,
not like I have any really unavoidable problems with this (I can
refactor but sometimes it just looks ugly). It was more of a
theoretical problem...

I'm not sure when I first encountered this, but only recently it did
hit me again, when I was writing special() for late macro. Because at
first I wanted to write it like this:

    public special(expr : PExpr) : bool
      | PExpr.This => true
      | PExpr.Base => true
      | _ when(Util.QidOfExpr(expr) is Some((id, name))) =>
        def ctx = name.GetEnv(env)
        if(ctx.LookupType(id) is Some(_))
          true
        else if(ctx.LookupMacro(id) is Some(_))
          true
      | _ => false

This way it was just looking better and I felt it to be more editable
in future, but then I found it doesn't work and remembered I can't use
variable bindings in when. I understand why this doesn't work and why
it might be (almost) impossible to do it the way I dream, but still...
It's just "wouldn't it be good if" thing then...