Question

What does block in Ruby mean? It looks similar with Smalltalk, but you can’t send messages to it.

For example, in smalltalk:

[:x | x + 3] value: 3

returns 6. But in ruby:

{|x| x + 3}.call 3

will cause SyntaxError.

Well, you can pass messages to lambda in ruby, though:

irb(main):025:0> ->(x){x+3}.call 3
=> 6

So in Ruby, block is not a block, but lambda is a block? Is this true? I mean, are there any differences between ruby lambda and smalltalk block? If this is true, then what is a ruby block?

Update:

From the comment and answer below, together with some googling, I guess I
have more understanding of Ruby block. In Ruby, usually a piece of code evaluates an value, and every value is an object. But, block doesn’t evaluate an value. So it’s not an object. Instead it can act as part of an object. For example, in {|x| x + 3} can act as a part of the object proc {|x| x + 3 }.

But it did confuse me. In smalltalk, almost every expression can be divided into objects (binding to variables are exceptions). It seems in Ruby, there are more exceptions.

Answer 1

First and the most important thing that Ruby block isn’t: an object. It is a syntactic construct, and also obviously has an equivalent implementation – but it is not an object, and thus can’t receive messages. Which makes your example of

{|x| x + 3}.call 3

ungrammatical. Lambdas, procs – those are objects that wrap a block, and have a call method which executes the block.

Thus, a block is simply a piece of code which can be passed to a method, outside the argument list – no more, no less. If you pass it to Proc.new constructor, for example, it will wrap it and give you an object you can handle:

Proc.new {|x| x + 3}.call 3