Re: Passing a named function instead of a code block?

On 3/21/09, 7stud -- <bbxx789_05ss@xxxxxxxxx> wrote:
James Coglan wrote:
2009/3/20 7stud -- <bbxx789_05ss@xxxxxxxxx>

&method(:square1)
grab a method as an object without calling it, we need to use
preceded by some symbol.
Yes, it probably looks that way. To see the difference it helps to know
how
Ruby is parsed. :square1 is an atomic unit representing the symbol whose
name is 'square1'. ":" is not an operator, it is part of the syntax for
symbols. However, "&" is an operator responsible for casting between
procs
and blocks. The expression '&square1' should be read '&( square1 )',
that is
we call square1 and cast the result of that using '&'. The same applies
to
'method'. 'method' is a method that takes a symbol/string and returns
the
Method object with that name in the current scope. 'method(square1)'
would
be interpreted as a call to square1, passing the result to 'method'.

So, '&square1' throws an error because you're calling a method with
insufficient arguments. '&:square1' would try to cast a symbol to a
proc,
which if you're using ActiveSupport would return the block { |object|
object.square1 }.

'&square2' is fine as square2 is just a variable referring to a proc.
Likewise, '&method(:square1)' is fine because method(:square1) is a
Method
object, which can be cast to a block.

Thanks for the explanation. You said that (&) casts between block and
Procs. But (&) also appears to be casting a Method to a block in this
line:

puts c.collect(&method(:fib))

Actually, its casting a Method to a Proc (by calling to_proc on it)
and then treating the proc as a block. To see that's what is
happening, try this:

class Integer
def to_proc
Proc.new { self }
end
end

(1..10).map(&1)

Can (&) cast a block to a Method?

No. & in a method call always treats a the given proc as a block given
to the method while & in a method definition always makes a block
given to the method as a Proc. Its probably best not to think of this
as "casting" except insofar as the first one "casts" to a proc first
by calling to_proc; blocks aren't objects, and if you do this:

def consume(object=nil)
if block_given? then yield else object end
end

p = lambda { puts "foo!" }
consume(&p)

You don't get the results you' d get if an object of the
(non-existent) class "Block" was passed to consume, you get the
results you would get if consume was called with a block. So thinking
of & as "casting" an argument to a different class in the method call
is misleading.

.

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