Strict v.s. Non-Strict
- Evaluation Order
- Strict Evaulation: evalulate the arguments, then call the function
- Non-strict Evaulation: call the function and pass the un-evaluted arguments in (somewhat like strings)
void fun(int x) { print fun, print x }
int arg() { print arg; return 2; }
fun(arg()); // Strict eval prints: arg fun 2,
// Non-strict eval prints: fun arg 2
Non-strict Evaulation
Non-strict Evaulation does not ever evaluate arguments it does not use
void fun(int x) { print fun } // doesn't USE x
int arg() { print arg; return 2; }
fun(arg()); // Strict eval prints: arg fun 2,
// Non-strict eval prints: fun (never uses x, never calls fun)
Parameter Passing w/ Non-strict Evaulation
Why can it memoize?? w/ no side effects, can assume referential transparency; foo(3) always gives same answer.
blah() { print blah } // w/ side effects
qux(x,y,z) { return x+y+z }
qux(blah(), blah(), blah());
floop(x) { return x+x+x+x+x+x+x }
floop(blah());
|
Call by Name prints |
Call by Need prints |
| qux(blah(), blah(), blah()); |
blah blah blah |
blah |
| floop(blah()); |
blah blah blah blah blah blah |
blah |
Short circuit
Do left of boolean op. If answer known, skip doing right. How can we tell? Side effects!!
bar() { print "bar"; return true; } // printing is a side effect
|
w/ short circuit |
w/o short circuit |
| true and bar() |
Must run right anyway, bar |
Must run right anyway, bar |
| false and bar() |
silence |
bar |
| true or bar() |
silence |
bar |
| false or bar() |
Must run right anyway, bar |
Must run right anyway, bar |
Watch out | and & are NOT boolean ops. Short circuit is NOT relevant to them!
