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Lisp Interpretter

Overview

This code implements a parser, interpretter, and REPL for a simple norvig.com-inspired Lisp.

It uses parboiled2 for parsing. Upon a successful parse, an net.parser.AST.SExpr instance gets returned.

Lastly, net.interpretter.LispParser#evaluate's return type is Either[(LispError, M), (MValue, M)].

How to Run this LISP REPL

sbt console

And then run the following to kick off the REPL:

scala> net.repl.LispRepl.runForever(Map())
res4: scalaz.effect.IO[Unit] = scalaz.effect.IOFunctions$$anon$6@2ae85ad9
 
scala> res4.unsafePerformIO
>(+1 2)
Right((3,Map()))
>
Right((3,Map()))
>4
Right((4,Map()))
>5
Right((5,Map()))
>100
Right((100,Map()))
>"foobar"
Right(("foobar",Map()))
>ff
Left((NoVarExists,Map()))
>((1))
Left((ProcError,Map()))
>(define x 100)
Right(((),Map(x -> 100)))
>x
Right((100,Map(x -> 100)))
>(define y 100)
Right(((),Map(x -> 100, y -> 100)))
>(> 100 9 8 7)
Right((true,Map(x -> 100, y -> 100)))
>(> x y)
Right((false,Map(x -> 100, y -> 100)))
>(+ x y )
Right((200,Map(x -> 100, y -> 100)))
>(+ 100 200) (define y 10)
Left((ParseError(ParseError(Position(12,1,13), Position(12,1,13), <2 traces>)),Map(x -> 100, y -> 100)))

lambda

Note - the lambda's values do not have to exist until execution time.

scala> net.repl.LispRepl.runForever(Map()).unsafePerformIO
>(define f (lambda (x) (+ x x)))
Complete(Right((Lambda,Map(f -> <function1>))))
>(f 20)
Complete(Right((40,Map(f -> <function1>))))
>(define g (lambda (x) (+ x z)))
Complete(Right((Lambda,Map(f -> <function1>, g -> <function1>))))
>(g 30)
Complete(Left((NoVarExists,Map(f -> <function1>, g -> <function1>))))
>(set! z 300)
Complete(Right(((),Map(f -> <function1>, g -> <function1>, z -> 300))))
>(g 30)
Complete(Right((330,Map(f -> <function1>, g -> <function1>, z -> 300))))

Using set! with lambda

>(set! x ((lambda (y) (+ y y)) 3))
Complete(Right((Val(SetOp(x,6)),Map(x -> Val(6)))))
>x
Complete(Right((Val(6),Map(x -> Val(6)))))
>x

Testing

There are two Scalatest tests:

  • net.parser.ParserTest
  • net.interpretter.LispInterpretterTest

TODO

  • show actual text when user types it in the REPL
  • use monadic chaining (rather than pattern matching)