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|
grammar LambdaCalculus
include Arithmetic
rule program
expression more_expressions:(';' space expression)* {
def eval(env={})
env = env.clone
last_eval = nil
expressions.each do |exp|
last_eval = exp.eval(env)
end
last_eval
end
def expressions
[expression] + more_expressions.elements.map {|elt| elt.expression}
end
}
end
rule expression
definition / conditional / application / function / super
end
rule definition
'def' space variable space expression {
def eval(env)
env[variable.name] = expression.eval(env)
end
}
end
rule conditional
'if' space '(' space condition:expression space ')' space
true_case:expression space 'else' space false_case:expression {
def eval(env)
if condition.eval(env)
true_case.eval(env)
else
false_case.eval(env)
end
end
}
end
rule primary
application / super
end
rule application
operator space expression <Application> {
def eval(env={})
left_associative_apply(operator.eval(env), env)
end
def left_associative_apply(operator, env)
if expression.instance_of?(Application)
expression.left_associative_apply(operator.apply(expression.operator.eval(env)), env)
else
operator.apply(expression.eval(env))
end
end
def to_s(env={})
operator.to_s(env) + ' ' + expression.to_s(env)
end
}
end
rule operator
function / variable
end
rule non_application
function / variable
end
rule function
'\\' param:variable '(' body:expression ')' {
class Closure
attr_reader :env, :function
def initialize(function, env)
@function = function
@env = env
end
def apply(arg)
function.body.eval(function.param.bind(arg, env))
end
def to_s(other_env={})
"\\#{function.param.to_s}(#{function.body.to_s(other_env.merge(env))})"
end
end
def eval(env={})
Closure.new(self, env)
end
def to_s(env={})
eval(env).to_s
end
}
end
rule variable
!keyword (
super {
def bind(value, env)
env.merge(name => value)
end
def to_s(env={})
env.has_key?(name) ? env[name].to_s : name
end
}
)
end
rule keyword
('if' / 'else') !non_space_char
end
rule non_space_char
![ \n] .
end
rule space
[ \n]*
end
end
|
