80 lines
2.4 KiB
Haskell
80 lines
2.4 KiB
Haskell
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module LambdaCalculus.Parser (parse) where
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import Control.Applicative (liftA2)
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import Control.Monad (void)
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import Text.Parsec hiding (parse)
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import qualified Text.Parsec as Parsec
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import Text.Parsec.String (Parser)
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import LambdaCalculus.Representation.AbstractSyntax
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-- | Parse a keyword, unambiguously not a variable name.
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keyword :: String -> Parser ()
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keyword kwd = try $ do
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void $ string kwd
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-- TODO: rephrase this in terms of `extension`
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notFollowedBy alphaNum
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-- | The extension of a variable name.
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-- The first letter of a variable name must be a letter,
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-- but the rest of the variable name may be more general.
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extension :: Parser String
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extension = many alphaNum
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-- | A variable name, e.g. `x`, `foo`, `f2`, `fooBar27`.
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name :: Parser String
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name = do
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notFollowedBy anyKeyword
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liftA2 (:) letter extension
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where
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anyKeyword = choice $ map keyword keywords
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where
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-- | Keywords that are forbidden from use as variable names.
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keywords = ["let", "in"]
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-- | A variable expression.
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variable :: Parser Expression
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variable = Variable <$> name
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-- | A lambda abstraction.
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abstraction :: Parser Expression
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abstraction = do
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char 'λ' <|> char '\\' ; spaces
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variables <- variableList ; spaces
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char '.' ; spaces
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body <- expression
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return $ Abstraction variables body
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where variableList :: Parser [String]
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variableList = name `sepBy` spaces
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-- | A function application.
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application :: Parser Expression
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application = Application <$> applicationTerm `sepEndBy` spaces
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where
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-- | An application term is any expression which consumes input,
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-- i.e. anything other than an ungrouped application.
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applicationTerm :: Parser Expression
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applicationTerm = let_ <|> variable <|> abstraction <|> grouping
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where
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-- | An expression grouped by parentheses.
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grouping :: Parser Expression
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grouping = between (char '(' >> spaces) (spaces >> char ')') expression
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-- | A `let` expression.
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let_ :: Parser Expression
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let_ = do
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keyword "let" ; spaces
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variable <- name ; spaces
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char '=' ; spaces
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value <- expression ; spaces
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string "in" ; spaces
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body <- expression
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return $ Let variable value body
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-- | Any expression.
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expression :: Parser Expression
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expression = application
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-- | Parse a lambda calculus expression.
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parse :: SourceName -> String -> Either ParseError Expression
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parse = Parsec.parse (expression <* eof)
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