Re: WG: Recovering the once true meaning of type NIL in LOOP

[Bruno H. <br...@cl...>]

Hi Don,

> Bruno Haible writes:
>  > > Consider (macroexpand-1'
>  > > (LOOP for (i j k) of-type (fixnum) across #())
>  > 
>  > Who says that this should expand to something reasonable?
>  > 
>  > Citing 6.1.1.7 "Destructuring"
> 
>  >   "When aligning the trees, an atom in the tree of type specifiers
>  >    that matches a cons in the variable tree declares the same type
>  >    for each variable in the subtree rooted at the cons."
>  > 
>  > So this form declares I as being of type FIXNUM and J and K as
>  > being of type NIL.
> 
> My interpretation of
>  an atom in the type specifier matches a cons in the variable tree
> would be fixnum matching (i j k) in
>  (LOOP for (i j k) of-type fixnum ...)
> so in that case i,j,k would all be declared fixnum,

Yes.

> but in (LOOP for (i j k) of-type (fixnum) ...)
> I would expect the atom fixnum to match only i,

Yes, agree.

> and I would NOT
> expect of-type (fixnum) to mean the same as of-type (fixnum nil nil)
> so I would expect j and k to be of unspecified type.

Here you have assumed that the type specifier has a list structure
with some kind of "optional" semantics. However, 6.1.1.7 is clear about:

  A type specifier for a destructuring pattern is a tree of type specifiers ...

with

  tree n. 1. a binary recursive data structure made up of conses and atoms

So the type specifier (FIXNUM) has to be read/interpreted as (FIXNUM . NIL).

Bruno