Customize variant int representation

tsnobip · November 26, 2020, 12:21pm

Since variants without payloads are compiled to int, is it possible to change the number used to represent a given variant with an annotation, like @bs.as?

As far as I know, it was possible with @bs.deriving(jsConverter), but it’s not even documented anymore and it required a conversion, which is not ideal.

tsnobip · November 30, 2020, 7:46am

Has anyone got any idea?

jfrolich · November 30, 2020, 11:07am

I wish this was something built into ocaml because I could use it for c-interop as well in a native project I am working on

jacobp100 · November 30, 2020, 11:07am

You can use polyvars that get compiled to strings.

type example = [#some | #variants]
let example = #some // compiled to "some"

tsnobip · November 30, 2020, 5:38pm

yes, the string use case is covered, it would be nice to have the same thing for ints!

Hongbo · December 1, 2020, 4:59am

this is a reasonable feature request, we will evaluate how complex on the implementation side

jacobp100 · December 1, 2020, 9:25am

Actually, thinking about it, it’d be super handy for me too. I have a few modules that do this mapping, but it ain’t pretty. I’d get a really nice code-size reduction with this.

My 2c if this was implemented, it’d be nice if this also worked with types that have variants with data.

github.com

jacobp100/technicalc-core/blob/master/packages/technicalc-editor/src/Encoding/Encoding_BuildElementMapping.re

let%private makeSureThisIsTheLastIndex = 79;
let%private toUint = (element: AST.t) =>
  switch (element) {
  /* Most common (make var-int encoding more efficient) */
  | N0_S => 0
  | N1_S => 1
  | N2_S => 2
  | N3_S => 3
  | N4_S => 4
  | N5_S => 5
  | N6_S => 6
  | N7_S => 7
  | N8_S => 8
  | N9_S => 9
  | DecimalSeparator => 10
  | Add => 11
  | Mul => 12
  | Sub => 13
  | Div => 14
  | OpenBracket => 15

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jfrolich · December 1, 2020, 10:06am

@jacobp100: Technicalc looks amazing. Did you create the UI using React-Native with ReScript?

jacobp100 · December 1, 2020, 11:42am

The UI is React Native with TypeScript. I personally find TS is a better fit for React at the moment (but these things can change). But ReScript works great for everything in the core package - and it probably comes out to 50/50 split between TS code and ReScript code overall for the whole project

Hongbo · December 3, 2020, 1:32pm

what’s the generated code output? I remember that I have some paths to optimize such cases

jacobp100 · December 3, 2020, 2:44pm

@Hongbo Yeah, the code generated is huge for this. It looks like I might be able to make stuff a bit easier for myself if I use the deriving(jsConverter). But currently, I actually get the output, encode it as JSON, then import it -

(Edit - I can’t use bs.deriving, because I need some constructors that contain values - and it looks like that’s not supported)

github.com

jacobp100/technicalc-core/blob/master/packages/technicalc-editor/run-mapping.js

const path = require("path");
const fs = require("fs");

const files = [
  {
    in: "src/Encoding/Encoding_BuildElementMapping.bs",
    out: "src/Encoding/Encoding_ElementMapping.js",
  },
  {
    in: "src/Encoding/Encoding_BuildUnitMapping.bs",
    out: "src/Encoding/Encoding_UnitMapping.js",
  },
  {
    in: "src/Encoding/LegacyEncoding_BuildElementMapping.bs",
    out: "src/Encoding/LegacyEncoding_ElementMapping.js",
  },
];

files.forEach((file) => {
  // eslint-disable-next-line

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github.com

jacobp100/technicalc-core/blob/master/packages/technicalc-editor/src/Encoding/Encoding_ElementMapping.js

export const mapping = [17,27,33,11,34,35,26,36,28,37,38,40,10,31,14,43,44,46,48,49,50,22,12,68,15,69,73,13,16,29,30,41,24,51,0,1,2,3,4,5,6,7,8,9,58,59,60,62,63,64,72,75,23,77,25,18,65,21,32,39,45,74,20,42,61,66,70,76,19,47,53,56,57,67,71,78,52,79,54,55];
export const reverseMapping = [34,35,36,37,38,39,40,41,42,43,12,3,22,27,14,24,28,0,55,68,62,57,21,52,32,54,6,1,8,29,30,13,58,2,4,5,7,9,10,59,11,31,63,15,16,60,17,69,18,19,20,33,76,70,78,79,71,72,44,45,46,64,47,48,49,56,65,73,23,25,66,74,50,26,61,51,67,53,75,77];

github.com

jacobp100/technicalc-core/blob/master/packages/technicalc-editor/src/Encoding/Encoding_Element.re

open AST;

[@bs.module "./Encoding_ElementMapping"]
external mapping: array(int) = "mapping";
[@bs.module "./Encoding_ElementMapping"]
external reverseMapping: array(t) = "reverseMapping";

let toUint = (element: t): int =>
  Belt.Array.getExn(mapping, Obj.magic(element));
let ofUint = (index: int): option(t) =>
  Belt.Array.get(reverseMapping, index);

Hongbo · December 4, 2020, 2:44am

Hi @jacobp100, if you shuffle the type definition of Ast_types.t a little bit, it should generate much better code, let me know if not.

type t = 
  | NS_0
  | NS_1 
  ..