Lo, on Sunday, January 13, Erik Steffl did write:
> type is a propert of variable.
Not exclusively. Two counter-examples, one in C, and one in Scheme.
C:
int x;
x = "foo";
You'll get a type error here at compile time, for obvious reasons.
Question: how can this be a type error if only variables have types?
You need to realize that "foo" has type (const) char * before you can
determine that you can't assign it to an int.
Scheme:
(define f
(lambda (x)
(cond
((boolean? x) (if x 42 23))
((symbol? x) (string-length (symbol->string x)))
((char? x) (char->integer x))
((vector? x) (vector-length x))
((procedure? x) (x 42))
((list? x) (length x))
((pair? x) (car x))
((number? x) (- x))
((string? x) (string-length x))
((port? x) (read x))
((promise? x) (force x)))))
This defines a function f with one argument, x. What's x's type? The
function is equally well-defined for an argument of just about any value
supported by R5RS, the current spec. (For that matter, what's the
return type of f? Answer: it's usually a number, but it depends on x!)
In languages like Scheme, Lisp, Python, and Smalltalk, almost all
typechecking is deferred to run-time. Therefore, it is meaningless to
describe the type of a variable; in these languages, types only apply to
values.
Richard
