link allof
April 28, 1990; Robert J. Alexander
Requires: co-expressions
This file is in the public domain.
allof{expr1,expr2} -- Control operation that performs iterative
conjunction.
Iterative conjunction permits a conjunction expression to be built
at run time which supports full backtracking among the created terms
of the expression. The computed expression can be of arbitrary
length, and is built via an iterative loop in which one term is
appended to the expression (as if connected with a "&" operator) per
iteration.
Expr1 works like the control expression of "every-do"; it controls
iteration by being resumed to produce all of its possible results.
The allof{} expression produces the outcome of conjunction of all of
the resulting instances of expr2.
For example:
global c
...
pattern := "ab*"
"abcdef" ? {
allof { c := !pattern ,
if c == "*" then move(0 to *&subject - &pos + 1) else =c
} & pos(0)
}
This example will perform a wild card match on "abcdef" against
pattern "ab*", where "*" in a pattern matches 0 or more characters.
Since pos(0) will fail the first time it is evaluated, the allof{}
expression will be resumed just as a conjunction expression would,
and backtracking will propagate through all of the instances of
expr2; the expression will ultimately succeed (as its conjunctive
equivalent would).
Note that, due to the scope of variables in co-expressions,
variables shared between expr1 and expr2 must have global scope,
hence c in the above example must be global.
The allof{} procedure models Icon's expression evaluation
mechanism in that it explicitly performs backtracking. The author of
this procedure knows of no way to invoke Icon's built-in goal
directed evaluation to perform conjunction of a arbitrary number of
computed expressions (suggestions welcome).