Maybe monad for crystal

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1

What would be the maybe monad for crystal.
Here for other languages,
http://www.rosettacode.org/wiki/Monads/Maybe_monad

2

Unless I’m misunderstanding what the Maybe Monad is, Crystal uses Nil unions for this. For example, the return type of Array(T)#min? is T | Nil (which is the same as T?) because there’s no minimum if the Array is empty. So, in that case, T | Nil is the same as a hypothetical type Maybe(T).

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3

Exactly. In Crystal map is Object#try. As an example, the code presented in the rosettacode for Ruby:

class Maybe
  def initialize(value)
    @value = value
  end
 
  def map
    if @value.nil?
      self
    else
      Maybe.new(yield @value)
    end
  end
end

Maybe.new(3).map { |n| 2*n }.map { |n| n+1 }
#=> #<Maybe @value=7>
 
Maybe.new(nil).map { |n| 2*n }.map { |n| n+1 }
#=> #<Maybe @value=nil>
 
Maybe.new(3).map { |n| nil }.map { |n| n+1 }
#=> #<Maybe @value=nil>

Becomes in Crystal:

3.try { |n| 2*n }.try { |n| n+1 }
#=> 7
 
nil.try { |n| 2*n }.try { |n| n+1 }
#=> nil
 
3.try { |n| nil }.try { |n| n+1 }
#=> nil

Look ma, no wrapper! :smile:

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Why doesn't Crystal leave the niche? StackOverflow 2022 Survey Result.
4

@Alain, since you brought the topic, do you want to post the example in rosettacode? (I’ll do it otherwise).

5

If you do, post the link and I’ll include it in the Rosetta Code list. :smiley:

3 Likes
6

A real Maybe ensures that Just and None are always disjoint, so Maybe(T) = T | Nil doesn’t suffice as T is nilable. A real implementation might be:

record Just(T), value : T
record None
# alias Maybe(T) = Just(T) | None

module Enumerable(T)
  def find2(& : T ->) : Just(T) | None
    each do |elem|
      return Just.new(elem) if yield elem
    end
    None.new
  end
end

def show(x)
  case x
  in Just; p x.value
  in None; puts "(none)"
  end
end

show [nil, 1].find2 { true }  # => nil
show [nil, 1].find2 { false } # => (none)

Actual monadic operations tend to manifest as noise rather than a benefit in a language like Crystal, which I imagine is why the standard library isn’t based on monads. There are probably some shards out there that provide Maybe and other true monads.

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7

Fair enough, but if you stack up Maybes, in most cases you don’t care where the None comes from (otherwise, you’ll use a more informative monad).