Harmon Nine wrote:
Something I don't understand:
In this example (exercise 6, p. 108 of the CTM):
-------------------
proc {Test X}
case X of f(a Y c) then {Browse 'case'(1)}
else {Browse 'case'(2)} end
end
-------------------
This suspends until X is bound:
-------------------
declare X Y {Test f(X Y d)}
-------------------
Why would this be, when it is clear that the 'd' in the input argument
will not match the 'c' in the "Test" procedure, and thus the
pattern-match will fail?
Why does pattern matching not "look ahead" like entailment?
A case statement only looks at one variable at a time.
Here, it is X. If you translate your example into the
kernel language, you get:
case X of f(A Y C) then
case A of a then
case C of c then ...
end
...
...
end
The case statement is designed this way for
simplicity and efficiency. It makes the case a very
nice primitive operation and also very efficient to
implement. If you want the 'look ahead' you have
to do a unification or an entailment check explicitly.
It's smarter but also more costly to implement.
Peter
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