Re: macros vs. blocks

[Noel Welsh <address@hidden>]

--- Bruce Lewis <brlewis@alum.mit.edu> wrote:
> A simple example that does #2 and #3 together is
> BRL's inputs syntax.
> 
> (inputs foo bar baz)
> 
> is equivalent to CGI.pm's
> foo=param('foo');
> bar=param('bar');
> baz=param('baz');

Interesting.  I have a similar macro that is a bit
more verbose and a bit more powerful.  It's definition
is:

  (let-bindings bindings
    ((name [mapping]) ...))

  bindings is an alist of strings, representing form  
       parameters

  name is the name of the variable and it's form 
       binding as in the inputs macro

  mapping is an optional mapping that transforms the
       value specified in the bindings.  mapping is
         string 
         | (list-of (cons (union string symbol) any)) 
         | string -> any

       if mapping is a string, it is the default value
       bound to name if no value is given in the 
       bindings.

       if mapping is an alist the value is that 
       associated with the string value of the
binding. 
       the symbol default is interpreted specially in
       the obvious way

       if mapping is a function the result is bound to
       name

It seems complicated in prose.  Code should help:

(define bindings '(("a" "foo") ("b" "2"))

(let-bindings bindings
  ((c "cee")
   (b (lambda (str) (string->number str)))
   (a '(("foo" foo) (default missing))))
  (list a b c))

evaluates to

  ('foo 2 "cee")

I present this as another good example of what can be
done with macros.  The equivalent in, say, OO style is
rather more tedious.

Noel

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