Implementing Read typeclass where parsing strings includes "$"

Question

I've been playing with Haskell for about a month. For my first "real" Haskell project I'm writing a parts-of-speech tagger. As part of this project I have a type called Tag that represents a parts-of-speech tag, implemented as follows:

data Tag = CC | CD | DT | EX | FW | IN | JJ | JJR | JJS ...

The above is a long list of standardized parts-of-speech tags which I've intentionally truncated. However, in this standard set of tags there are two that end in a dollar sign ($): PRP$ and NNP$. Because I can't have type constructors with $ in their name, I've elected to rename them PRPS and NNPS.

This is all well and good, but I'd like to read tags from strings in a lexicon and convert them to my Tag type. Trying this fails:

instance Read Tag where
    readsPrec _ input =
        (\inp -> [((NNPS), rest) | ("NNP$", rest) <- lex inp]) input

The Haskell lexer chokes on the $. Any ideas how to pull this off?

Implementing Show was fairly straightforward. It would be great if there were some similar strategy for Read.

instance Show Tag where
    showsPrec _ NNPS = showString "NNP$"
    showsPrec _ PRPS = showString "PRP$"
    showsPrec _ tag  = shows tag

Answer 1

You're abusing Read here.

Show and Read are meant to print and parse valid Haskell values, to enable debugging, etc. This doesn't always perfectly (e.g. if you import Data.Map qualified and then call show on a Map value, the call to fromList isn't qualified) but it's a valid starting point.

If you want to print or parse your values to match some specific format, then use a pretty-printing library for the former and an actual parsing library (e.g. uu-parsinglib, polyparse, parsec, etc.) for the latter. They typically have much nicer support for parsing than ReadS (though ReadP in GHC isn't too bad).

Whilst you may argue that this isn't necessary, this is just a quick'n'dirty hack you're doing, quick'n'dirty hacks have a tendency to linger around... do yourself a favour and do it right the first time: it means there's less to re-write when you want to do it "properly" later on.

Answer 2

Don't use the Haskell lexer then. The read functions use ParSec, which you can find an excellent introduction to in the Real World Haskell book.

Here's some code that seems to work,

import Text.Read
import Text.ParserCombinators.ReadP hiding (choice)
import Text.ParserCombinators.ReadPrec hiding (choice)

data Tag = CC | CD | DT | EX | FW | IN | JJ | JJR | JJS deriving (Show)

strValMap = map (\(x, y) -> lift $ string x >> return y)

instance Read Tag where
    readPrec = choice $ strValMap [
        ("CC", CC),
        ("CD", CD),
        ("JJ$", JJS)
        ]

just run it with

(read "JJ$") :: Tag

The code is pretty self explanatory. The string x parser monad matches x, and if it succeeds (doesn't throw an exception), then y is returned. We use choice to select among all of these. It will backtrack appropriately, so if you add a CCC constructor, then CC partially matching "CCC" will fail later, and it will backtrack to CCC. Of course, if you don't need this, then use the <|> combinator.