Two things out of one mechanism. `_' as a type means "work it out from
the initializer", so (var n _ (strlen s)) stops needing size-t spelled
out; `type-of' hands a macro the type of an expression, so a macro can
dispatch on what it was handed rather than on what was declared. Both
read the same answers from two sides.
Algorithm W's core, intra-procedural, with the extensions C forces:
- an unknown type, since (include stdio.h) brings in names we never
parsed. Unification is consistency rather than equality, so
anything touching an unparsed declaration stops constraining
instead of rejecting a program that compiled yesterday;
- the usual arithmetic conversions, since `+' is not a function of
one type;
- checking mode for initializers, since #(0 0) has no type of its own
and takes one from its context. #(T : ...) is the way out of that.
What it wanted on the way:
- what type a *name* has, which neither the typedef nor the tag
database recorded. One table serves functions and variables, since
a function type already has a surface spelling;
- a scope chain, so a (var c int 9) inside a do ends with the block;
- form-type, keyed by cons cell, so one form has one type;
- macros expanded during the walk rather than before it, so type-of
is answered in the scope the macro was written in.
Closures take the same machinery: a receiver whose type comes from a
call, captures written (name expr) and typed from the expression, and
conversion from a bare function wherever a closure is expected.
type-match grew `_' on the pattern side, since (closure ((int)) int)
and (closure ((float)) int) were separate clauses for one case.
alex-eg
1f445f0f9b