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haskell library to parse commandline args - an alternative to optparse-applicative

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# butcher

#### Chops a command or program invocation into digestable pieces.

Similar to the `optparse-applicative` package, but less features,
more flexibility and more evil.

The main differences are:

* Provides a pure interface by default

* Exposes an evil monadic interface, which allows for much nicer binding of
  command part results to some variable name.

In `optparse-applicative` you easily lose track of what field you are
    modifying after the 5th `<*>` (admittedly, i think -XRecordWildCards
    improves on that issue already.)

Evil, because you are not allowed to use the monad's full power in this
    case, i.e. there is a constraint that is not statically enforced.
    See below.

* The monadic interface allows much clearer definitions of commandparses
  with (nested) subcommands. No pesky sum-types are necessary.

## Examples

The minimal example is

~~~~.hs
main = mainFromCmdParser $ addCmdImpl $ putStrLn "Hello, World!"
~~~~

But lets look at a more feature-complete example:

~~~~.hs
main = mainFromCmdParserWithHelpDesc $ \helpDesc -> do

addCmdSynopsis "a simple butcher example program"
  addCmdHelpStr "a very long help document"

addCmd "version" $ do
    porcelain <- addSimpleBoolFlag "" ["porcelain"]
      (flagHelpStr "print nothing but the numeric version")
    addCmdHelpStr "prints the version of this program"
    addCmdImpl $ putStrLn $ if porcelain
      then "0.0.0.999"
      else "example, version 0.0.0.999"

addCmd "help" $ addCmdImpl $ print $ ppHelpShallow helpDesc

short <- addSimpleBoolFlag "" ["short"]
    (flagHelpStr "make the greeting short")
  name <- addStringParam "NAME"
    (paramHelpStr "your name, so you can be greeted properly")

addCmdImpl $ do
    if short
      then putStrLn $ "hi, " ++ name ++ "!"
      else putStrLn $ "hello, " ++ name ++ ", welcome from butcher!"
~~~~

Further:

- [Full description of the above example, including sample behaviour](example1.md)
- [Example of a pure usage of a CmdParser](example2.md)
- [Example of using a CmdParser on interactive input](example3.md)
- The [brittany](https://github.com/lspitzner/brittany) formatting tool is a
  program that uses butcher for implementing its commandline interface. See
  its [main module source](https://github.com/lspitzner/brittany/blob/master/src-brittany/Main.hs)
  or [the config flag parser](https://github.com/lspitzner/brittany/blob/master/src/Language/Haskell/Brittany/Config.hs).

## The evil monadic interface

As long as you only use Applicative or (Kleisli) Arrow, you can use the
interface freely. When you use Monad, there is one rule: Whenever you read
any command-parts like in

~~~~
f <- addFlag ...
p <- addParam ...
~~~~

you are only allowed to use bindings bound thusly in any command's
implemenation, i.e. inside the parameter to `addCmdImpl`. You are _not_
allowed to force/inspect/patternmatch on them before that. _good_ usage is:

~~~~
addCmdImpl $ do
  print x
  print y
~~~~

while _bad_ would be

~~~~
f <- addFlag
when f $ do
  p <- addParam
  -- evil: the existence of the param `p`
  -- depends on parse result for the flag `f`.
~~~~

That means that checking if a combination of flags is allowed must be done
after parsing. (But different commands and their subcommands (can) have
separate sets of flags.)

## (abstract) Package intentions

Consider a commandline invocation like "ghc -O -i src -Main.hs -o Main". This
package provides a way for the programmer to simultaneously define the
semantics of your program based on its arguments and retrieve documentation
for the user. More specifically, i had three goals in mind:

1. Straight-forward description of (sub)command and flag-specific behaviour
2. Extract understandable usage/help commandline documents/texts from that
   descriptions, think of `ghc --help` or `stack init --help`.
3. Extract necessary information to compute commandline completion results
   from any partial input. (This is not implemented to any serious degree.)

## Semantics

Basic elements of a command are flags, parameters and subcommands. These can
be composed in certain ways, i.e. flags can have a (or possibly multiple?)
parameters; parameters can be grouped into sequences, and commands can have
subcommands.

Commands are essentially `String -> Either ParseError out` where `out` can
be chosen by the user. It could for example be `IO ()`.

To allow more flexible composition, the parts of a command have the "classic"
parser's type: `String -> Maybe (p, String)` where `p` depends on the part.
Parse a prefix of the input and return something and the remaining input, or
fail with `Nothing`.

A command-parser contains a sequence of parts and then a number of subcommands
and/or some implementation.

### Commands and Child-Commands

- ~~~~ .hs
  myParser :: CmdParser Identity Int ()
  myParser = return ()
  ~~~~

input | `runCmdParserSimple input myParser`
  ----- | -------------
  "" | Left "command has no implementation"
  "x" | Left "error parsing arguments: could not parse input/unprocessed input at: \"x\"."

- ~~~~ .hs
  myParser :: CmdParser Identity Int ()
  myParser = do
    addCmd "foo" $ addCmdImpl 2
    addCmd "bar" $ addCmdImpl 3
    addCmd "noimpl" $ pure ()
    addCmd "twoimpls" $ do
      addCmdImpl 4
      addCmdImpl 5
    addCmdImpl 1
  ~~~~
  
  input | `runCmdParserSimple input myParser`
  ----- | -------------
  "" | Right 1
  "x" | Left "error parsing arguments: could not parse input/unprocessed input at: \"x\"."
  "foo" | Right 2
  "bar" | Right 3
  "foo bar" | Left "error parsing arguments: could not parse input/unprocessed input at: \"bar\"."
  "noimpl" | Left "command has no implementation"
  "twoimpls" | Right 5

### Flags

- without any annotation, no reodering is allowed and the flags must appear in order:
  ~~~~ .hs
  myParser :: CmdParser Identity (Bool, Int, Int) ()
  myParser = do
    b <- addSimpleBoolFlag "b" [] mempty
    c <- addSimpleCountFlag "c" [] mempty
    i <- addFlagReadParam "i" [] "number" (flagDefault 42)
    addCmdImpl $ (b, c, i)
  ~~~~
  
  input | `runCmdParserSimple input myParser`
  ----- | -------------
  "" | Right (False,0,42)
  "-b -c -i 3" | Right (True,1,3)
  "-c -b" | Left "error parsing arguments: could not parse input/unprocessed input at: \"-b\"."
  "-c -c -c" | Right (False,3,42)

- this time with reordering; also "j" has no default and thus becomes mandatory, still it must not
  occur more than once:
  ~~~~ .hs
  myParser :: CmdParser Identity (Bool, Int, Int, Int) ()
  myParser = do
    reorderStart -- this time with reordering
    b <- addSimpleBoolFlag "b" [] mempty
    c <- addSimpleCountFlag "c" [] mempty
    i <- addFlagReadParam "i" [] "number" (flagDefault 42)
    j <- addFlagReadParam "j" [] "number" mempty -- no default: flag mandatory
    reorderStop
    addCmdImpl $ (b, c, i, j)
  ~~~~
  
  input | `runCmdParserSimple input myParser`
  ---------------------------- | -------------
  "-b" | Left "error parsing arguments:<br>could not parse expected input -j number with remaining input:<br>InputString \"\" at the end of input."
  "-j=5" | Right (False,0,42,5)
  "-c -b -b -j=5" | Right (True,1,42,5)
  "-j=5 -i=1 -c -b" | Right (True,1,1,5)
  "-c -j=5 -c -i=5 -c" | Right (False,3,5,5)
  "-j=5 -j=5" | Left "error parsing arguments: could not parse input/unprocessed input at: \"-j=5\"."

- addFlagReadParams - these can occur more than once. Note that defaults have slightly different semantics:
  ~~~~ .hs
  myParser :: CmdParser Identity (Int, [Int]) ()
  myParser = do
    reorderStart
    i <- addFlagReadParam "i" [] "number" (flagDefault 42)
    js <- addFlagReadParams "j" [] "number" (flagDefault 50)
    reorderStop
    addCmdImpl $ (i, js)
  ~~~~
  
  input | `runCmdParserSimple input myParser`
  ---------------------------- | -------------
  "" | Right (42,[])
  "-i" | Left "error parsing arguments: could not parse input/unprocessed input at: \"-i\"."
  "-j=1 -j=2 -j=3" | Right (42,[1,2,3])
  "-j" | Right (42,[50])
  "-i=1" | Right (1,[])
  "-j=2" | Right (42,[2])
  "-j=2 -i=1 -j=3" | Right (1,[2,3])
  
### Params

TODO

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