Ruby's Enumerable #map and #select methods implemented with #inject

gistfile1.rb

# The idea is just to illustrate that Ruby's #map and #select Enumerable methods
# are simply specific flavours of a fold operation, which Ruby implements as #inject.
# I've implemented #map and #select using #inject, and at the end I implemented #inject
# recursively. Ruby does #inject with Array#each, for very good reason. The recursive 
# implementation is really just to illustrate the core Functional Programming concept 
# which it derives from.

# We'll start simple.  Map without applying a function:

[1,2,3,4,5].inject([]) {|memo, obj| memo << obj }

# => [1,2,3,4,5]



# Map that accepts a block argument (this can obv be implemented as an instance 
# method on Enumerable.  See #map.  :P ):

def my_map(list)
  list.inject([]) {|memo, obj| memo << yield(obj) }
end

my_map([1,2,3,4,5]) {|i| i * 2 }
# => [2, 4, 6, 8, 10]



# Select without a passed-in function (note how the recursive nature of the function
# is made clear by the return value of the block in the case where the predicate is not met):

[1,2,3,4,5].inject([]) {|memo, obj| obj > 3 ? memo << obj : memo }
# => [4,5]



# Select that accepts a function argument.  Also easy to implement as an instance method:

def my_select(list)
  list.inject([]) {|memo, obj| yield(obj) ? memo << obj : memo }
end

my_select([1,2,3,4,5]) {|i| i > 3 }
# => [4, 5]



# And just for fun lets implement inject recursively as #fold.  You wouldn't want to do 
# this in Ruby, the stack wouldn't handle long lists, and it destroys the list (pure 
# FP is side-effect free).  This simply illustrates the concept in Functional Programming, 
# which is where these methods come from. In a pure FP language one would implement any 
# method that acts on a list in this way (in relation to itself), but it wouldn't be as
# awkward to express.

def fold(current, list, &block)
  return current if list.empty?
  block.call(current, fold(list.shift, list, &block))
end

fold(0, [1,2,3,4]) {|memo, obj| memo + obj }
# => 10