class Amb
class ExhaustedError < RuntimeError; end
def initialize
@fail = proc { fail ExhaustedError, "amb tree exhausted" }
end
def choose(*choices)
prev_fail = @fail
callcc { |sk|
choices.each { |choice|
callcc { |fk|
@fail = proc {
@fail = prev_fail
fk.call(:fail)
}
if choice.respond_to? :call
sk.call(choice.call)
else
sk.call(choice)
end
}
}
@fail.call
}
end
def failure
choose
end
def assert(cond)
failure unless cond
end
end
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