How to build a string in ruby by calling block methods?

Question

What I want to do is define a method that gets the result of every method in a block (or calls every method in a block) and returns the resultant string...

For example:

def a_method(&block)
    ## build a string by calling each of the blocks methods
end

def brackets(str)
    '('+str+')'
end

a_method do
    brackets "foo"
    brackets "bar"
end

# desired output => (foo)(bar)

I tried putting yield in a_method, but as I expected that just returns the last evaluated expression from the block (in this case, (bar)).

I've looked around on how to build a string from calls inside a block, but can't find anything. I would have thought this would be quite a common technique.. could someone explain how?

PinnyM · Accepted Answer · 2012-05-15 19:00:22Z

2

You can use a DSL object to handle this, giving you more flexibility:

class StringBuilder
  def method_missing(method, *args, &block)
    @strings << (@original_self.send method, *args, &block)
  end

  def build(&block)
    @strings = []
    @original_self = eval "self", block.binding
    instance_eval &block
    @strings.join(' ') #format as you like
  end
end

Now use where needed:

sb = StringBuilder.new
sb.build do
  brackets "foo"
  brackets "bar"
end
#=> (foo) (bar)

answered May 15, 2012 at 19:00

PinnyM

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1 Comment

PinnyM Over a year ago

Just a caveat: because this relies on method_missing, in the unlikely event you try to build strings using methods already included in the Object class this won't work properly. You can have the StringBuilder inherit from BlankSlate to get around this if need be.

Victor Moroz · Accepted Answer · 2012-05-15 18:40:23Z

0

["foo", "bar"].map(&method(:brackets)).join

UPDATE Another way if you want to use various methods:

def a_method(&block)
    ## build a string by calling each of the blocks methods
    p block[].join
end

def brackets(str)
    '('+str+')'
end

a_method do
    [ brackets("foo"),
      brackets("bar") ]
end

edited May 15, 2012 at 18:40

answered May 15, 2012 at 18:34

Victor Moroz

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2 Comments

Alex Coplan Over a year ago

thanks, but the question isn't really about the output, it's about specifically using block syntax.

Alex Coplan Over a year ago

I've seen code that does it without putting it in a list.. because yes my goal is to use multiple methods

gmalette · Accepted Answer · 2012-05-15 18:45:11Z

0

If you stricly want to use blocks to create your string, you will have to use clever ways to remember it. One of those ways is to use a string instance and eval the block on it.

def a_method(&block)
  "".instance_eval(&block)
end

def brackets(str)
  self << "(#{str})"
end

str = a_method do
  brackets "foo"
  brackets "bar"
end

puts str
#=> (foo)(bar)

answered May 15, 2012 at 18:45

gmalette

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3 Comments

Alex Coplan Over a year ago

hmm.. could I somehow respond to method calls from the block in the method that handles the block? (a_method) - what I'm trying to achieve is HTML-generation using blocks as demonstrated on tryruby.org

Alex Coplan Over a year ago

looks like you're onto the right thing with instance_eval.... github.com/Sophrinix/TryRuby/blob/…

gmalette Over a year ago

I was typing up but you can just refer to PinnyM's answer

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