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module Patter
class Var; end
class Fun
def initialize
@patterns = {}
end
def when(*args, &block)
raise "Block arity does not match number of free variables" if args.count(Var) != block.arity
@patterns[args] = block
end
def call(*args)
free_vars, block = match(args)
block.call(*free_vars)
end
alias_method :[], :call
private
def match(args)
@patterns.each do |pattern, block|
next if pattern.length != args.length
free_variables = []
matched = true
pattern.zip(args).each do |pair|
if pair.first == Var then
free_variables << pair.last
next
end
if pair.first != pair.last then
matched = false
break
end
end
return [free_variables, block] if matched
end
raise "Inexhaustive patterns"
end
end
end
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