1. Short-circuit evaluation
- Posted by rforno at tutopia.com
Aug 09, 2002
Rob:
I know, it is seldom used and impractical, but have you thought about
extending short-circuit evaluation to the operations I mention below? I was
thinking about this only regarding educational purposes: you know there are
16 possible binary Boolean operations with 2 operands. Some comparison
operations can be used in a Boolean context. For example, XOR is not needed
at the variable-level because it may be substituted by !=. The operations I
was thinking of are:
if (a > b) >= (x = z) then ... If a > b is true, then there is no need to
evaluate x = z.
if (c <= u) > ( d > g) then... If c <= u is false, then there is no need to
evaluate d > g
and similarly with <= and <.
These operations, when used with Boolean operands, might be thought about as
"implication", etc.
= and != cannot be short-circuited.
AFAIK, C doesn't use this little trick.
Regards.
2. Re: Short-circuit evaluation
Ricardo Forno writes:
> if (a > b) >= (x = z) then ... If a > b is true, then there is no need to
> evaluate x = z.
True, but I don't think I've ever seen an expression like
that used in a real program.
Short-circuiting can only theoretically be performed in Euphoria
when an expression must produce an atom as a result.
So we know that:
true or expression is true (without evaluating expression)
false and expression is false (without evaluating expression)
If expression might yield a sequence, we can't short-circuit.
Short-circuit evaluation could be extended into new areas.
For example, the expression returned from a type...
type one_item(object x)
return atom(x) or length(x) = 1
end type
Right now, length(x) will fail if x is an atom.
With short circuiting, this expression would be ok,
and we can assume that a type must return an atom.
Other cases would include expressions that are assigned
to a variable (or parameter) declared as an atom.
However, people need to remember, preferably
without consulting the manual, which expressions
can be short-circuited and which can't.
I don't want to create a huge section in the manual,
filled with if's, and's and but's, that tries to explain
when short circuit evaluation will be used.
Right now, and/or short circuiting happens for
control-flow statements -if/elsif/while. In practice,
that covers most of the useful situations.
Regards,
Rob Craig
Rapid Deployment Software
http://www.RapidEuphoria.com
