[Question] How to propagate line number from lexer to parser

[leana8959]

Hello,

I'm writing a lexer and a parser for a custom language. I would like to propagate the line numbers of each token from the lexing stage (should it succeed) to the parsing stage, so that parsing errors can be more friendly.

The problem is, parsing tokens composed with SourcePos seems to be significantly harder. I can't instruct token what's expected (of type MyTokenType), because it wants a MyToken (the input type of my parser) which is the wrapped type with the line number. It doesn't make sense to include the number in the expected token.

Even if I were to use [TokenType] (the type without line number) as the input stream directly, other variants of the same issue crops up. For example: if I were to parse a sequence of TInt Int tokens, it doesn't make sense to say TInt 123 is expected, because any token of type TInt with any value that the lexer produces is valid.

Is there an easy way out of this, or is there a better, more idiomatic alternative ?

Lexer:

```hs -- Wrapped token type with position in the source file data MyToken = MyToken {tokenPos :: SourcePos, tokenType :: MyTokenType} deriving (Eq, Ord, Show) data MyTokenType = TInt Int deriving (Eq, Ord, Show) type Lexer = Parsec Void String -- Wrap with position withPos :: Lexer MyTokenType -> Lexer MyToken withPos p = MyToken <$> getSourcePos <*> p spaceConsumer :: Lexer () spaceConsumer = (skipMany . oneOf) ['\t', ' '] lexer :: Lexer [MyToken] lexer = do between spaceConsumer (spaceConsumer <* eof) (many (lInt <* spaceConsumer)) where lInt = withPos $ TInt . read <$> takeWhile1P Nothing isDigit ```

Parser:

```hs type Parser = Parsec Void [MyToken] -- Primitive over `token` to parse a lexical item token' :: (MyTokenType -> Maybe a) -- ^ Predicate -> MyTokenType -- ^ Expected -> Parser a token' p t = token (p . tokenType) (S.singleton . Tokens . NE.singleton $ t {- type error, expected `Token` but got `TokenType` -}) ```

Thank you for maintaining the library, megaparsec is awesome :)

[leana8959]

Update:

I found this gist, which shows that one can hack the VisualStream to make MyToken not show the embedded line information, and hack the TraversableStream to make line number aware of the token's actual position in the source file.

Though my original question of "how to instruct token what's expected" still remains. Unless it doesn't matter, since it won't be shown to the user, so we can create a dummy position?