ApplicativeDo language extension with `Parsing` applicative still looking for Monad instance

Question

I'm trying to write a parser using the parsers package using do syntax. Here is an example:

{-# LANGUAGE ApplicativeDo #-}
import Text.Parser.Char (string, spaces)
import Text.Parser.Token (TokenParsing, natural)

issueParser :: TokenParsing p => p Integer
issueParser = do
  spaces
  string "**Issue:**"
  spaces
  string "https://github.com" <|> string "github.com"
  string "/commercialhaskell/stack/issues/"
  natural

The error GHC gives me is Could not deduce (Monad p) arising from a do statement from the context: TokenParsing p. This error message is correct that TokenParsing does not provide Monad as a super class, however it does provide Applicative which means because I have this language extension turned on, I should be able to use the do syntax with just Applicative. What am I doing wrong/missing here?

score 14 · Accepted Answer · answered Feb 16 '17 at 21:47

14

Figured it out. To get this example to work on ghc 8.0.2 you would need to add underscore generators, like this:

{-# LANGUAGE ApplicativeDo #-}
import Text.Parser.Char (string, spaces)
import Text.Parser.Token (TokenParsing, natural)

issueParser :: TokenParsing p => p Integer
issueParser = do
  _ <- spaces
  _ <- string "**Issue:**"
  _ <- spaces
  _ <- string "https://github.com" <|> string "github.com"
  _ <- string "/commercialhaskell/stack/issues/"
  n <- natural
  pure n

There is already a ghc bug to address this here: https://ghc.haskell.org/trac/ghc/ticket/12666

answered Feb 16 '17 at 21:47

Asa

1,466
9
27

Oh wow, I am surprised to see that `ApplicativeDo` did not handle this rather obvious case! – Joachim Breitner Feb 16 '17 at 23:33
`ApplicativeDo` is quite dumb. E.g. it can't handle a `let` statement of any sort whatsoever – luqui Feb 17 '17 at 01:54
I had a similar problem: http://stackoverflow.com/questions/41210599/applicativedo-not-working-with-sequencing – danidiaz Feb 17 '17 at 05:20
Oh look at that. I'm surprised that this question didn't show up in my search. It is very similar – Asa Feb 17 '17 at 05:43

ApplicativeDo language extension with `Parsing` applicative still looking for Monad instance

1 Answers1