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:

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:
    could not parse expected input -j number with remaining input:
    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

Changes

Revision history for butcher

1.3.1.1 -- April 2018

  • Fixup version bound

1.3.1.0 -- April 2018

  • Add/Expose two more functions: addAlternatives and varPartDesc

1.3.0.1 -- April 2018

  • Support ghc-8.4
  • Drop support for ghc<8

1.3.0.0 -- February 2018

  • Experimental: Hidden commandparts (do not appear in help)
  • Experimental: Bash completion
  • Add addHelpCommandWith to support user-defined column count
  • Fix help document printing (ribbons)
  • Fix completion behaviour

1.2.1.0 -- November 2017

  • Fix bug in 'ppUsageWithHelp'
  • some utilities for interactive usage in new module UI.Butcher.Monadic.Interactive

1.2.0.0 -- October 2017

  • Rename some Monadic.Param.*, deprecate old versions.
    • addReadParam -> addParamRead
    • addReadParamOpt -> addParamReadOpt
    • addStringParam -> addParamString
    • addStringParamOpt -> addParamStringOpt
    • addStringParams -> addParamStrings
    • addRestOfInputStringParam -> addParamRestOfInput
  • Add functions addParamNoFlagString, addParamNoFlagStringOpt, addParamNoFlagStrings
  • Fix flag parsing behaviour (ignore initial spaces)

1.1.1.0 -- October 2017

  • Add addNullCmd function that descends into childcommand on an epsilon match
  • Add addStringParams function that reads all remaining words

1.1.0.2 -- September 2017

  • Improve 'usage' pretty-printing

1.1.0.1 -- August 2017

  • Adapt for ghc-8.2

1.1.0.0 -- May 2017

  • First version. Released on an unsuspecting world.
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