March 8, 2019

Implementing a Functional Language: Parsing Core

This is an appendix to a series in implementing a functional language. The introduction is here. This is a literate Haskell file - you can download the source here. To load it into GHCi and play around, you can use the following command (you’ll need the source for the Core.Language module as well):

stack --resolver lts-12.2         \
      ghci --package megaparsec   \
           --package text         \
      2019-03-03-parsing-core.lhs \
      2019-03-03-the-core-language.lhs

This module exports the parser for our language (parser). It can be passed to functions like Text.Megaparsec.parse to parse input into a Core.CoreProgram. We also re-export Text.Megaparsec.parse for convenience.

{-# LANGUAGE OverloadedStrings #-}
module Core.Parse (parser, parse) where
import Core.Language
import Text.Megaparsec
import qualified Text.Megaparsec.Char.Lexer as L
import Text.Megaparsec.Char
import Data.Text (Text)
import qualified Data.Text as T
import Data.Void
import Data.Proxy

Our parser is formed by combining together smaller parsers, in typical parser combinator style. In the interest of brevity, we put little effort into useful error messages. Here we define a type for all our parsers, which specifies a basic error type and an input type of Text. See the Megaparsec documentation for a description of the Parsec and ErrorFancy types.

type Parser = Parsec (ErrorFancy Void) Text

Lexing

Lexing is the process of converting a stream of characters to a stream of tokens. It’s typically used to strip out whitespace, comments, and other parts of the input that have no bearing on the parse result. To avoid having to process the input twice, we intersperse lexing with parsing by wrapping all primitive parsers in a space consumer which is responsible for stripping any trailing “space”. Our space consumer will strip whitespace and any lines beginning with --.

spaceConsumer :: Parser ()
spaceConsumer = L.space space1 (L.skipLineComment "--") empty
lexeme :: Parser a -> Parser a
lexeme = L.lexeme spaceConsumer 

Parsing Primitives

symbol and num are our two primitive parsers - we’ll build everything else out of them. symbol parses a specific string, and num parses a number.

symbol :: Text -> Parser Text
symbol = L.symbol spaceConsumer
num :: Parser Int
num = lexeme L.decimal

We define some helpers that we’ll rely on later on. between is a parser combinator which wraps a parser in a “left” and “right” parser which will parse either side of the input.

semicolon :: Parser Text
semicolon = symbol ";"
parens :: Parser a -> Parser a
parens = between (symbol "(") (symbol ")")
angles :: Parser a -> Parser a
angles = between (symbol "<") (symbol ">")

A Core identifier (i.e. a variable name, supercombinator name or let binding name) consists of alphanumeric characters. The first character can’t be a number.

identifier :: Parser Text
identifier = lexeme $ do
  first <- letterChar
  rest <- many alphaNumChar
  pure $ tokensToChunk (Proxy :: Proxy Text) (first : rest)

A Core program is a collection of supercombinator definitions.

parser :: Parser CoreProgram
parser = supercombinator `sepBy1` semicolon

A supercombinator is a name, an optional list of arguments, and a expression representing the function body.

supercombinator :: Parser CoreScDefn
supercombinator = do
  name <- var
  args <- many var
  symbol "="
  body <- expr
  pure (name, args, body)

A variable name is either an identifier or one of the built-in operators. It can’t be a reserved keyword.

var :: Parser Name
var = choice $ operators ++ [try alphaVar]
      where alphaVar = do
              v <- identifier
              if v `elem` keywords
              then
                fail . T.unpack $ "cannot use keyword " <> v <> " as variable"
              else
                pure v
operators :: [Parser Text]
operators = map symbol ["==", "!=", ">=", "<=", "+", "-", "*", "/", ">", "<"]
keywords :: [Text]
keywords = ["let", "letrec", "case", "in", "of", "Pack", "->"]

An expression is either a let(rec), a case, an application, a constructor, a numeric literal or a variable. To avoid the problem of left recursion we handle applications separately from simple expressions. If we didn’t do this, parsing f x would recurse infinitely on f.

expr :: Parser CoreExpr
expr = let_ <|> case_ <|> application <|> aexpr
aexpr :: Parser CoreExpr
aexpr = parens expr <|> constructor <|> fmap ENum num <|> fmap EVar var
application :: Parser CoreExpr
application = foldl1 EAp <$> some aexpr
let_ :: Parser CoreExpr
let_ = do
  isRec <- letrec
  defns <- letDefn `sepBy1` semicolon
  symbol "in"
  e <- expr
  pure $ ELet isRec defns e
    where letrec = (symbol "letrec" >> pure Recursive) <|> (symbol "let" >> pure NonRecursive)
          letDefn = do
            v <- var
            symbol "="
            e <- expr
            pure (v, e)
  
case_ :: Parser CoreExpr
case_ = do
  symbol "case"
  scrutinee <- expr
  symbol "of"
  alts <- caseAlternatives
  pure $ ECase scrutinee alts
caseAlternatives :: Parser [Alter Name]
caseAlternatives = alt `sepBy1` semicolon
  where alt = do
          tag <- angles num
          args <- many var
          symbol "->"
          e <- expr
          pure (tag, args, e)
constructor :: Parser CoreExpr
constructor = do
  symbol "Pack{"
  tag <- num
  symbol ","
  arity <- num
  pure $ EConstr tag arity

This is all there is to the parser. Like the printer, it is short and sweet.