Crystal How to check if the block argument is given inside the function

问题

Suppose a function defined like this:

def composition(text : String, k : Int32) : Array(String)
  kmers = Array(String).new
  (0 .. text.size - k).each do |i|
    kmers << text[i, k]
    yield text[i, k]
  end
  return kmers
end

How do I check if the block argument is given inside the function? If the block argument is given, kmers will be yielded. If not given, kmers will be returned as an array of strings.

回答1:

Such a check is impossible, because a method accepting a block (using yield anywhere) simply has a different signature. But that also means that you don't need the check, just make 2 methods like this:

# if you want to be explicit (makes no difference):
# def composition(text : String, k : Int32, &block)
def composition(text : String, k : Int32)
  (0 .. text.size - k).each do |i|
    yield text[i, k]
  end
end

def composition(text : String, k : Int32) : Array(String)
  kmers = [] of String
  composition(text, k) do |s|
    kmers << s
  end
  return kmers
end

来源：https://stackoverflow.com/questions/39190854/crystal-how-to-check-if-the-block-argument-is-given-inside-the-function