Hello,
Consider this cond example:
10 20 30
{ { [ dup 30 = ] [ + + ] }
{ [ 2dup + 100 > ] [ - - ] }
{ [ 3dup + + 70 = ] [ * * ] }
{ [ t ] [ / / ] }
} cond
With cond, each predicate has to be careful to maintain the stack for the next
branch. If the expression portion of the predicate has an inferable effect,
the computer should be able to do this work for us. It would be nice if we
could just write:
10 20 30
{ { [ 30 = ] [ + + ] }
{ [ + 100 > ] [ - - ] }
{ [ + + 70 = ] [ * * ] }
{ [ t ] [ / / ] }
} switch
Indeed we can... Here's the switch macro:
MACRO: switch ( quot -- )
[ [ preserving ] [ ] bi* ] assoc-map
[ , cond ]
bake ;
Here I am showing off the bi* spread combinator. 'preserving' is a cool word:
it takes a predicate and turns it into a quotation which "preserves" it's
arguments.
( scratchpad ) 5 6 [ > ] preserving call
----------
5
6
f
----------
Let's see what 'preserving' does to a quot.
( scratchpad ) [ > ] preserving
----------
[ [ > ] 2 nkeep 3 nrot ]
----------
( scratchpad )
Here it is:
: preserving ( predicate -- quot )
dup infer effect-in
dup 1+ swap rot
[ , , nkeep , nrot ]
bake ;
Ed
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