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|
#!/usr/bin/ruby
class Value < Struct.new(:value)
def to_s
value.to_s
end
def inspect
"<#{self}>"
end
def reducible?
false
end
end
class Number < Value
end
class Boolean < Value
end
class Variable < Struct.new(:name)
def to_s
name.to_s
end
def inspect
"<#{self}>"
end
def reduce(environment)
environment[name]
end
def reducible?
true
end
end
class BiOp < Struct.new(:left, :right)
def inspect
"<#{self}>"
end
def reducible?
true
end
end
class Add < BiOp
def to_s
"#{left} + #{right}"
end
def reduce(environment)
if left.reducible?
Add.new(left.reduce(environment),right)
elsif right.reducible?
Add.new(left,right.reduce(environment))
else
Number.new(left.value+right.value)
end
end
end
class Multiply < BiOp
def to_s
"#{left} * #{right}"
end
def reduce(environment)
if left.reducible?
Add.new(left.reduce(environment),right)
elsif right.reducible?
Add.new(left,right.reduce(environment))
else
Number.new(left.value*right.value)
end
end
end
class LessThan < BiOp
def to_s
"#{left} < #{right}"
end
def reduce(environment)
if left.reducible?
LessThan.new(left.reduce(environment), right)
elsif right.reducible?
LessThan.new(left, right.reduce(environment))
else
Boolean.new(left.value < right.value)
end
end
end
class DoNothing
def to_s
'do-nothing'
end
def inspect
"<#{self}>"
end
def ==(other_statement)
other_statement.instance_of?(DoNothing)
end
def reducible?
false
end
end
class Assign < Struct.new(:name, :expression)
def to_s
"#{name} = #{expression}"
end
def inspect
"<#{self}>"
end
def reducible?
true
end
def reduce(environment)
if expression.reducible?
[Assign.new(name, expression.reduce(environment)), environment]
else
[DoNothing.new, environment.merge({name => expression })]
end
end
end
class If < Struct.new(:condition, :consequence, :alternative)
def to_s
"if (#{condition}) { #{consequence} } else { #{alternative} }"
end
def inspect
"<#{self}>"
end
def reducible?
true
end
def reduce(environment)
if condition.reducible?
[If.new(condition.reduce(environment), consequence, alternative), environment]
else
case condition
when Boolean.new(true)
[consequence, environment]
when Boolean.new(false)
[alternative, environment]
end
end
end
end
class Sequence < Struct.new(:first, :second)
def to_s
"#{first}; #{second}"
end
def inspect
"<#{self}>"
end
def reducible?
true
end
def reduce(environment)
case first
when DoNothing.new
[second,environment]
else
reduced_first, reduced_environment = first.reduce(environment)
[Sequence.new(reduced_first, second), reduced_environment]
end
end
end
class Machine < Struct.new(:statement, :environment)
def step
self.statement, self.environment = statement.reduce(environment)
end
def run
while statement.reducible?
puts "#{statement}, #{environment}"
step
end
puts "#{statement}, #{environment}"
end
end
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