problem combining partially overlapping parsers with Alternative

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Assessment

Difficulty
4/5
Estimated time
3-5 days
Newbie friendliness
35/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
haskell
Domain
cli

Research direction

Start by compiling and running the supplied Haskell program, then inspect Options.Applicative.Internal and the Alternative behavior for the two parser orderings. Done means both orderings handle ./foo a, ./foo --bar a b, and ./foo b --bar a without an incorrect Missing: --bar ARG failure.

Written by the indexing model from the issue text.

Description

The test program below prints this --help

Usage: foo ([--bar ARG] [string] | [string])

And "./foo --bar a b" works. However:

joey@darkstar:~>./foo a
Missing: --bar ARG

Seems that in bar <|> foo, the bar parser can fail in a way that makes it never try the
foo parser, which would succeed if it were tried.

If it's changed to foo <|> bar, the example above works, as does "./foo --bar a b" , so that version of the program works ok. Except, "./foo b --bar a" does not work; normally the bar
parser would accept that, but now the foo parser fails in a way that makes the bar parser not work.

import Options.Applicative
import Options.Applicative.Internal

data Foo = Foo [String] | Bar String String
        deriving (Show)

parser :: Parser Foo
parser = bar <|> foo
  where
        foo = Foo <$> many (argument str ( metavar ""))
        bar = Bar <$> strOption (long "bar") <*> argument str (metavar "dir")

main = print =<< execParser opts
  where
         opts = info (parser <**> helper)
                ( fullDesc
                <> progDesc "testcase"
                <> header "testcase"
                )
Dominant language
Haskell
Stars
957
Forks
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