On Sat, Nov 21, 2009 at 5:30 PM, Seebs [email protected] wrote:
1
Is it actually guaranteed that the output will have “a” after “b” and “c”?
In C, it wouldn’t be. (Compilers are free to evaluate the operands in any
order whatsoever, then feed them into operators, all that precedence and
grouping controls is which operands go to which operators.)
I think you are confusing precedence ordering with evaluation strategy.
Precedence determines the order in which the
operators/functions/methods are performed. so in
a + b * c
the * is performed before the +
But the timing of when the argument expressions is evaiuated is a
different issue.
Ruby like most other languages uses what is called applicative
ordering, which means that the arguments to a function are evaluated
before the function is called (or applied). They don’t need to be
evaluated JUST before the function is applied, just BEFORE it is.
Consider this Ruby program and its output:
class NoisyNum
def initialize(name)
@name = name
puts “#{name} created”
end
def to_s
@name.to_s
end
def +(other)
NoisyNum.new(“#{self}+#{other}”)
end
def (other)
NoisyNum.new("#{self}#{other}")
end
end
def N(name)
NoisyNum.new(name)
end
x = N(:a) + N(:b) * N(:c)
a created
b created
c created
bc created
a+bc created
A few languages use what is called Normal order (strange since it’s
not the normal strategy used) in which the evaluation of the arguments
is delayed until they are needed by a function. Some languages like
Lisp/Scheme normally use applicative order, but have what are called
special forms which use normal order.
This allows special forms to be used to define control flow like
if/then/else to be implemented with a function with the condition, and
the two alternatives.
In Ruby you could think of the expression a || b as a special form in
that the b expression is not evaluated unless a is not truthy.
HTH
–
Rick DeNatale
Blog: http://talklikeaduck.denhaven2.com/
Twitter: http://twitter.com/RickDeNatale
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