Add a "dhall parse-tree" command for debugging purposes

[sjakobi]

Sample output:

$ echo 'toMap {=} : a ? b' | dhall parse-tree
ToMap 
    ( RecordLit ( fromList [] ) ) 
    ( Just 
        ( ImportAlt 
            ( Var ( V "a" 0 ) ) 
            ( Var ( V "b" 0 ) )
        )
    )
$ echo "http://a?/" | dhall parse-tree
Embed 
    ( Import 
        { importHashed = ImportHashed 
            { hash = Nothing
            , importType = Remote 
                ( URL 
                    { scheme = HTTP
                    , authority = "a" 
                    , path = File 
                        { directory = Directory { components = [] }
                        , file = "" 
                        } 
                    , query = Just "/" 
                    , headers = Nothing
                    } 
                )
            } 
        , importMode = Code
        } 
    )

[Gabriella439]

I'm a little bit reluctant to do this because the internal AST is highly volatile and verbose (especially if we start adding Maybe Src fields for preserving comments). I don't want end users to depend too much on it from the command line.

Also, remember that the CBOR representation already plays the role of a language-independent AST and dhall encode --json lets you debug that:

$ dhall encode --json <<< 'toMap {=} : a ? b'
[
    27,
    [
        8,
        {}
    ],
    [
        3,
        11,
        [
            "a",
            0
        ],
        [
            "b",
            0
        ]
    ]
]

$ dhall encode --json <<< 'http://a?/'
[
    24,
    null,
    0,
    0,
    null,
    "a",
    "",
    "/"
]

[sjakobi]

Personally I don't find the output of dhall encode --json very easy to understand.

Regarding your compatibility concerns – the usage info already says "for debugging":

Output the parsed syntax tree (for debugging)

Would it help if I add "output may change at any time" or something like that?

[sjakobi]

I wonder why Hydra complains about a missing aeson dependency when the executable with the aeson dependency is configured to be unbuildable (with -f-buildexample):

setupCompileFlags: -package-db=/tmp/nix-build-pretty-simple-3.1.0.0.drv-0/setup-package.conf.d -j4 -threaded
[1 of 1] Compiling Main             ( Setup.hs, /tmp/nix-build-pretty-simple-3.1.0.0.drv-0/Main.o )
Linking Setup ...
configuring
configureFlags: --verbose --prefix=/nix/store/3s0hq9wnjhyzwq31jf68fbp2imfpxrw4-pretty-simple-3.1.0.0 --libdir=$prefix/lib/$compiler --libsubdir=$pkgid --docdir=/nix/store/5s3zghpy8qdyqhxys23h8j779mlxn707-pretty-simple-3.1.0.0-doc/share/doc/pretty-simple-3.1.0.0 --with-gcc=gcc --package-db=/tmp/nix-build-pretty-simple-3.1.0.0.drv-0/package.conf.d --enable-split-objs --enable-library-profiling --disable-executable-profiling --enable-shared --disable-coverage --disable-executable-dynamic --disable-tests --enable-library-vanilla --enable-library-for-ghci -f-buildexample -f-buildexe --extra-lib-dirs=/nix/store/w8bxy3xrvrd1ci5772y7arkicj0q7x3h-ncurses-6.1/lib --extra-lib-dirs=/nix/store/d60s5bwv6r3hl6w8cg5j7znwaiva43n8-gmp-6.1.2/lib
Warning: pretty-simple.cabal: Ignoring unknown section type: custom-setup
Configuring pretty-simple-3.1.0.0...
Setup: At least the following dependencies are missing:
aeson -any, optparse-applicative -any
builder for '/nix/store/syia6wg6dk64mkxgppbqb6yqm9l8bv8g-pretty-simple-3.1.0.0.drv' failed with exit code 1

[Gabriella439]

@sjakobi: Would it be possible to hide the parse-tree subcommand from the --help output? The reason I ask is that I don't want to publicize Haskell-specific features for a utility which is designed to be language-agnostic

I also think the subcommand should have Haskell in the name, since it is language-specific (e.g. haskell-tree)

[Gabriella439]

@sjakobi: That seems like a cabal bug to me. However, it might be simpler to just let it build the sample executables

[sjakobi]

Would it be possible to hide the parse-tree subcommand from the --help output?

I think hiding should be possible, but I haven't gotten it to work yet (see pcapriotti/optparse-applicative#365).

[sjakobi]

That seems like a cabal bug to me. However, it might be simpler to just let it build the sample executables

The error was already present when I didn't use the cabal flags: https://hydra.dhall-lang.org/build/45291/nixlog/3

Am I using cabal2nix incorrectly? It just doesn't seem to include the executable dependencies:

$ cabal2nix cabal://pretty-simple
{ mkDerivation, ansi-terminal, base, Cabal, cabal-doctest
, containers, criterion, doctest, Glob, mtl, QuickCheck, stdenv
, template-haskell, text, transformers
}:
mkDerivation {
  pname = "pretty-simple";
  version = "3.1.0.0";
  sha256 = "ea8a062fe7a535a0778ea0f85442551b9a67fad95979fc3e4c617399452c775c";
  isLibrary = true;
  isExecutable = true;
  setupHaskellDepends = [ base Cabal cabal-doctest ];
  libraryHaskellDepends = [
    ansi-terminal base containers mtl text transformers
  ];
  testHaskellDepends = [
    base doctest Glob QuickCheck template-haskell
  ];
  benchmarkHaskellDepends = [ base criterion text ];
  homepage = "https://github.com/cdepillabout/pretty-simple";
  description = "pretty printer for data types with a 'Show' instance";
  license = stdenv.lib.licenses.bsd3;
}

[Gabriella439]

@sjakobi: What version of cabal2nix are you using? I just tested and I'm getting the executable dependencies:

$ cabal2nix cabal://pretty-simple-3.1.0.0
{ mkDerivation, aeson, ansi-terminal, base, bytestring, Cabal
, cabal-doctest, containers, criterion, doctest, Glob, mtl
, optparse-applicative, QuickCheck, stdenv, template-haskell, text
, transformers
}:
mkDerivation {
  pname = "pretty-simple";
  version = "3.1.0.0";
  sha256 = "ea8a062fe7a535a0778ea0f85442551b9a67fad95979fc3e4c617399452c775c";
  isLibrary = true;
  isExecutable = true;
  setupHaskellDepends = [ base Cabal cabal-doctest ];
  libraryHaskellDepends = [
    ansi-terminal base containers mtl text transformers
  ];
  executableHaskellDepends = [
    aeson base bytestring optparse-applicative text
  ];
  testHaskellDepends = [
    base doctest Glob QuickCheck template-haskell
  ];
  benchmarkHaskellDepends = [ base criterion text ];
  homepage = "https://github.com/cdepillabout/pretty-simple";
  description = "pretty printer for data types with a 'Show' instance";
  license = stdenv.lib.licenses.bsd3;
}

[Gabriella439]

I forgot to mention that the version I'm using is:

$ cabal2nix --version
cabal2nix 2.8.1

[sjakobi]

$ cabal2nix --version
cabal2nix 2.15.0

For other packages I'm still getting the executableHaskellDepends…

I've made a bug report: NixOS/cabal2nix#436.

[Gabriella439]

Just one minor suggestion