On Mon, Nov 15, 2010 at 8:45 AM, Shaping <shap...@charter.net> wrote:
> I'm also thinking that the stack-effect data could be more binding than it
> currently is. I noticed while compiling some new words that the compiler
> will infer a stack-effect, and tell you when you've written it incorrectly.
> This looks almost binding. So my suggestion/question is: Can we use the
> stack effects to check at least the superficial (type- and count-based)
> correctness of a forth expression, as it is assembled on a Listener command
> line?
The stack effects are already binding. When you enter a word
definition, and the compiler determines that the code's stack effect
does not match the declared effect, you'll get an immediate compiler
error. The one exception to this is "inline" higher-order words (words
such as "map" that take a quotation as input)—for these words, stack
checking will be deferred to call sites that use the inline
higher-order word, because the stack checking can't be done thoroughly
without a quotation to fill in the parameter. If the word declares a
static stack effect for its quotations, the effect will be enforced on
the caller:
--
( scratchpad ) : foo ( x quot: ( x -- ) -- ) call ; inline
( scratchpad ) : bar ( x y -- ) [ + . ] foo ;
! "bar" should cause an "Input quotation to foo doesn't match its
expected effect" error
--
If the word declares a polymorphic effect (using "..a" style stack
variables to represent a variable number of values), the arities of
the stack variables will be unified, and an error raised if
unification fails:
--
( scratchpad ) \ if see
: if ( ..a ? true: ( ..a -- ..b ) false: ( ..a -- ..b ) -- ..b )
? call ;
( scratchpad ) : bas ( x y z? -- ) [ + ] [ neg ] if ;
! "bas" should cause an "Input quotations to “if” don't match their
expected effects" error
--
If a higher-order word declares no stack effect for its quotation
arguments, the overall stack effect of the caller will still be
checked when the body of the higher-order word and its arguments have
been inlined.
-Joe
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