From: Brian Candler <b.candler@...>
Date: 2009-01-11T03:11:55+09:00
Subject: Re: functional programming

Pascal J. Bourguignon wrote:
> Are you saying that you must use lambda everywhere, even when you have a
> named function?  That named functions are less than anonymous functions?

No. I was trying: (1) to understand what the underlying problem was that 
you were trying to solve in jumping through all these hoops, rather than 
using lambda { ... } directly, and (2) to understand Mike's assertion 
that Ruby lambdas are not first-class functions (i.e. "The difference 
between lambda { } and a real first-class function is quite profound").

>> smallest = lambda { |list|
>>   smallestElement[list[1..-1], list.first]
>> }
> 
> There's a difference between (def smallest(x) ; x ; end)
> and (smallest = (lambda { |x| x }))
> 
> In the former case, you can write (smallest [1,2,3])
> in the later you can't:

Sure, in the latter you'd write smallest[[1,2,3]]. The outer brackets 
delimit the call of the lambda, and the inner ones mark the array.

> irb(main):003:0> (smallest = (lambda { |x| x }))
> (smallest = (lambda { |x| x }))
> #<Proc:0x0035fdf4@(irb):3>
> irb(main):004:0> (smallest [1,2,3])
> (smallest [1,2,3])
> (irb):3: warning: multiple values for a block parameter (3 for 1)
>   from (irb):4
> [1, 2, 3]

As far as I can see, it is only accidental that this runs at all. 
'smallest' is not a method, it's a local variable, so smallest[...] can 
only be interpreted as a call to the #[] method on that object, i.e. 
smallest.call(...)

> As you show it, it is perfectly possible to do it this way in ruby.  But
> it's more complex. You have to know now what this & syntax does.  You
> have to wonder why you cannot write:
> 
> 
> (def biggest(x)
>   ...
> end)
> 
>  [
>    [1],
>    [1,1,1,1],
>    [1,2,3,4],
>    [4,3,2,1],
>    [1,2,3,4,3,2,1],
>    [4,3,2,1,2,3,4]].map(&biggest)
> 
>                                                                                                                                                  ArgumentError: 
> wrong number of arguments (0 for 1)
>   from (irb):17:in `biggest'
>   from (irb):17

I would have expected map(&method(:biggest)) to work, but unfortunately 
it doesn't due to arity reasons.

I guess you rob Peter to pay Paul. In Ruby, a bareword like "biggest" 
can be a method name (in which case it invokes the method, and evaluates 
to its return value), or a variable name (in which case it evaluates to 
the content of that variable).

This means that common cases in Ruby don't need any syntactic markers 
like () to say "invoke this method". The code just looks cleaner. But in 
that case, if you want to refer to the *name* of the method or the 
*method/function itself* then you need to mark in that case instead. 
i.e. :foo or method(:foo)
-- 
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