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144 lines
2.9 KiB
144 lines
2.9 KiB
require_relative "error"
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require_relative "expr"
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module Lox
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class ParseError < Error
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def initialize(token, message)
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at = token.type == :EOF ? "end" : "'#{token.lexeme}'"
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super(token.line, "at #{at}", message)
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end
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end
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class Parser
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class State < Struct.new(:tokens, :current)
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def initialize(tokens)
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super(tokens, 0)
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end
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def expression = equality
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def equality
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expr = comparison
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while match?(:BANG_EQUAL, :EQUAL_EQUAL)
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op = prev
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right = comparison
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expr = Expr::Binary.new(expr, op, right)
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end
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expr
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end
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def comparison
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expr = term
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while match?(:GREATER, :GREATER_EQUAL, :LESS, :LESS_EQUAL)
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op = prev
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right = term
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expr = Expr::Binary.new(expr, op, right)
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end
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expr
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end
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def term
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expr = factor
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while match?(:MINUS, :PLUS)
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op = prev
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right = factor
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expr = Expr::Binary.new(expr, op, right)
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end
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expr
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end
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def factor
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expr = unary
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while match?(:SLASH, :STAR)
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op = prev
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right = unary
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expr = Expr::Binary.new(expr, op, right)
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end
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expr
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end
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def unary
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return primary unless match?(:BANG, :MINUS)
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op = prev
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right = unary
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Expr::Unary.new(op, right)
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end
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def primary
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return Expr::Literal.new(false) if match?(:FALSE)
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return Expr::Literal.new(true) if match?(:TRUE)
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return Expr::Literal.new(nil) if match?(:NIL)
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return Expr::Literal.new(prev.literal) if match?(:NUMBER, :STRING)
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if match?(:LEFT_PAREN)
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expr = expression
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consume!(:RIGHT_PAREN, "Expect ')' after expression.")
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return Expr::Grouping.new(expr)
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end
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raise ParseError.new(peek, "Expect expression.")
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end
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private
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def match?(*types)
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return false unless check?(*types)
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advance!
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return true
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end
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def consume!(type, message)
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raise ParseError.new(peek, message) unless check?(type)
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advance!
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end
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def check?(*types)
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return false if eot?
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types.include?(peek.type)
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end
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def advance!
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self.current += 1 unless eot?
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prev
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end
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def eot? = peek.type === :EOF
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def peek = tokens.fetch(current)
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def prev = tokens.fetch(current - 1)
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def synchronize!
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advance!
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until eot?
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return if prev.type == :SEMICOLON
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return if %i[CLASS FUN VAR FOR IF WHILE PRINT RETURN].include?(peek.type)
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advance!
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end
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end
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end
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# In the book, this returns nil when there's an error, but
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# that feels weird so let's move that error handling up the
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# stack for now.
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def parse(tokens)
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state = State.new(tokens, 0)
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state.expression
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end
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end
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end
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