#--
# merlin/let.rb : merlin-let 0.0.1
#
# Copyright (C) 2004  Merlin Hughes  <ruby AT merlin DOT org>
#
# This library is free software; you can redistribute it and/or
# modify it under the terms of the GNU Lesser General Public
# License as published by the Free Software Foundation; either
# version 2.1 of the License, or (at your option) any later version.
#
# This library is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser General Public
# License along with this library; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307  USA
#
#++
# Lisp/Scheme-like let mixin for IO, Hash and ENV.
#
# Disclaimer: I know neither Lisp, Scheme nor Ruby.
#
# io.let(io) { |io| block }
# - Reopens the IO object with the parameter,
#   executes the block and then restores the
#   IO object. A dup of the original IO is
#   passed as a parameter to the block.
#
#   STDERR.let(STDOUT) { |stderr|
#     STDERR.puts("I'm going to STDOUT")
#     stderr.puts("I'm going to STDERR")
#   }
#   STDERR.puts("I'm going to STDERR")
# 
# hash.let(array) { |array| block }
# - array is a [key, value] pair or an array of [key, value] pairs
# - Reassigns the specified entries in the hash
#   to the specified values, or removes the entry
#   if the pair has no second element, executes
#   the block, and then restores the hash
#   entries. The original values are passed
#   to the block in the same format as they
#   are passed to the let method.
#
# ENV.let(array) { |array| block } # array is a pair or an array of pairs
# - array is a [key, value] pair or an array of [key, value] pairs
# - Ditto.
#   
#  ENV.let([ [ 'PATH' ], [ 'HOME', '/tmp' ] ]) { |originals|
#    # PATH is removed, and HOME is /tmp
#  }
#  # PATH and HOME are restored

module IOLet
  # TODO: I could redefine reopen similarly.
  def let(io)
    original = dup
    begin
      reopen(io)
      yield(original)
    ensure
      reopen(original)
    end
  end
end

module HashLet
  def let(values)
    originals = []
    begin
      simple = values.first.is_a?(Array)
      (simple ? values : [ values ]).each { | pair |
        key, value = pair
        originals << (has_key?(key) ? [ key, self[key] ] : [ key ])
        (pair.length > 1) ? (self[key] = value) : delete(key)
      }
      yield(simple ? originals : originals.first)
    ensure
      originals.reverse_each { | pair |
        key, value = pair
        (pair.length > 1) ? (self[key] = value) : delete(key)
      }
    end
  end
end

class IO
  include IOLet
end

class Hash
  include HashLet
end

ENV.extend HashLet