From: Dmitry Vazhov <dmitryelastic@...>
Date: 2009-12-18T15:51:13+09:00
Subject: Re: Suggestion of Array#=== which improves case/when behavio

"Set" class has meaning close to "Range" class. If we will define 
Set#=== as

class Set
  def ===( arg )
    self.any?{|template| template === arg  }
  end
end

we will be able to write case/when code in haskell-like pattern-matching 
style. here is an example of simple s-expression evaluator from my page:

Sexp = Set[Array, Numeric]    # means - array or number
Boolean = Set[true, false]
Sexpbool = Set[Array] | Boolean  # means -  array or true or false


def evale(e)  # function which will evaluate S-Expression
  case e
    when Numeric, Boolean then e
    when [:-, Sexp] then    - evale(e[1])
    when [:-, Sexp, Sexp] then evale(e[1]) - evale(e[2])
    when [:+, Sexp, Sexp] then evale(e[1]) + evale(e[2])
    when [:*, Sexp, Sexp] then evale(e[1]) * evale(e[2])
    when [:**, Sexp, Sexp] then evale(e[1]) ** evale(e[2])
    when [:>, Sexp, Sexp] then evale(e[1]) > evale(e[2])
    when [:if, Sexpbool, Sexp, Sexp] then evale(e[1]) ?evale(e[2]) : 
evale(e[3])
    when Object then fail("something went wrong")
  end
end

def test_exp_eval
  exp = [:*, [:-, 9, 2], [:+, 8, [:-, 2]]] # ->  (9 - 2) * (2 + 4) = 42
  assert_equal 42, evale(exp)
  exp2 = [:if, true, 10, 20]
  assert_equal 10, evale(exp2)
  exp3 = [:if, [:>, [:**, 5, 5], 4000], 1, 2] # -> 2 , because 4000 > 
5**5
  assert_equal 2, evale(exp3)
end





Dmitry Vazhov wrote:
> Hello,
> 
> We already have :=== operator defined in Module, Range, Regexp and Proc
> which is great for case/when statement.  For all other objects :=== mean
> :==.
> 
> My suggestion is to extend Array with === method:
> 
> Original behaviour:
> [1,2,3] === [1,2,3] #=> true
> [1,2,3] === [1,2,4] #=> false
> [1,2,3] === 2       #=> false
> 
> Adding code:   -- this code is similar to Array#== , but uses :=== for
> comparing each element
> class Array
>   def ===( arg )
>     arg.is_a?(Array) && self.size == arg.size && (0...size).all?{|i|
> self[i] === arg[i] }
>   end
> end
> 
> After adding code:
> [1,2,3] === [1,2,3] #=> true
> [1,2,3] === 2       #=> false
> 
> [1,2,Object] === [1,2,3] #=> true
> [1,2,/^some/] === [1,2,"some string"] #=> true
> [1,2,[Symbol, Array]] === [1,2,[:key, []]] #=> true       <----   this
> is an example of deep structure matching
> 
> You can see other examples at
> http://github.com/dmitryelastic/dumb-patterns-matching/blob/master/patterns_matching.rb
> and   http://github.com/dmitryelastic/dumb-patterns-matching

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