Expose dhall as javascript library via ghcjs WIP

[Profpatsch]

I plan on playing a bit with compiling the compiler to JS, depending on the free-time I have.

A first (trivial) success:

On current nixpkgs master, nix-build -A haskell.packages.ghcjsHEAD.dhall gives us node binaries that basically work (you have to split the shebang line of the binary manually, then it works).

philip@katara ~/nixpkgs (current-build)> ~/tmp/dhall
{ foo = "bar" }   
{ foo : Text }

{ foo = "bar" }
philip@katara ~/nixpkgs (current-build)> file ~/tmp/dhall
/home/philip/tmp/dhall: a /nix/store/gbladqk26m3mhmaj147lfv5rpsiilfxb-nodejs-slim-6.11.5/bin/node script, UTF-8 Unicode text executable, with very long lines
philip@katara ~/nixpkgs (current-build)> head /nix/store/z8kb2pgilq0083van0faqghkj21p2m2q-dhall-1.8.0/bin/dhall
#!/nix/store/gbladqk26m3mhmaj147lfv5rpsiilfxb-nodejs-slim-6.11.5/bin/node var h$currentThread = null;
var h$stack = null;
var h$sp = 0;
var h$initStatic = [];
var h$staticThunks = {};
var h$staticThunksArr = [];
var h$regs = [];
var h$r1 = 0;
var h$r2 = 0;
var h$r3 = 0;
philip@katara ~/nixpkgs (current-build)> 

[Gabriel439]

Yeah, I already test drove using GHCJS on Dhall. If I remember correctly, the main issue I ran into was that the import system didn't work for URLs, but I suppose that it's still useful if people want to "Try Dhall" in the browser. The main limitation will be the absence of the Prelude

[Profpatsch]

The main limitation will be the absence of the Prelude

Since we already put the whole Haskell runtime as JS into the browser, supplying the Prelude should not be that much overhead, right?
I’d love to expose the library parts to Purescript and reuse the typechecker, parser and awesome error reporting. Maybe by going through yet another json export-/import step it’s possible get something like (very non-performant) Purescript import.

[Gabriel439]

Oh yeah, I forgot that we support customizable import resolution, so this should be doable if we use the Haskell API translated to JavaScript

[Profpatsch]

Now we just need somebody to implement a zero-copy data view from GHCJS output to Purescript structures. :P

[Profpatsch]

So, I’ve had some success and set up a repo.

philip@katara ~/c/h/dhall-js (master)> 
nix-build -I nixpkgs=/home/philip/nixpkgs release.nix  --option build-use-substitutes false -A small
/nix/store/mxlcim7c9binajhgny02my3fp3lzy310-export-test-small
philip@katara ~/c/h/dhall-js (master)> nix-shell -p nodejs --run "node ./result"
Object (fromList [("bar",String "Hello DhallJS!"),("foo",Number 23.0)])

^C⏎                           

philip@katara ~/c/h/dhall-js (master)> 
nix-build -I nixpkgs=/home/philip/nixpkgs release.nix  --option build-use-substitutes false -A small
/nix/store/mxlcim7c9binajhgny02my3fp3lzy310-export-test-small
philip@katara ~/c/h/dhall-js (master)> 
du -ha /nix/store/mxlcim7c9binajhgny02my3fp3lzy310-export-test-small
624K	/nix/store/mxlcim7c9binajhgny02my3fp3lzy310-export-test-small
philip@katara ~/c/h/dhall-js (master)> 
nix-build -I nixpkgs=/home/philip/nixpkgs release.nix  --option build-use-substitutes false -A orig
/nix/store/zmp0qarqnd3c1i7yva0jmfypn89smmv8-export-test
philip@katara ~/c/h/dhall-js (master)> 
du -ha /nix/store/zmp0qarqnd3c1i7yva0jmfypn89smmv8-export-test
2.3M	/nix/store/zmp0qarqnd3c1i7yva0jmfypn89smmv8-export-test

[Gabriel439]

Nice job! :)

[Gabriel439]

This is fixed by dhall-lang/dhall-haskell#734