Add missing pure annotations

[OliverJAsh]

We also need to add pure annotations to the classes defined in index.js, however we can't add the pure annotation in the correct position because these are transpiled to IIFEs.

For example:

monocle-ts/src/index.ts

Line 1465 in 9bd7fa6

export class Setter<S, A> {

This will become:

var Setter = /** @class */ (function () {
    function Setter() {
    }
    return Setter;
}());
export { Setter };

The pure annotation needs to be added like so:

-var Setter = /** @class */ (function () {
+var Setter = /** @class */ /*#__PURE__*/ (function () {

I believe this can be solved by using a Babel plugin that adds the pure annotation to output of TypeScript. Alternatively, we can disable class transpilation.

---

Pure annotations are very easy to miss and hard to get right, so perhaps we should automate this across the fp-ts ecosystem, using something like one of these tools. WDYT @gcanti?

• https://github.com/Andarist/babel-plugin-annotate-pure-calls
• https://github.com/morlay/babel-plugin-pure-calls-annotation

[OliverJAsh]

I believe this can be solved by using a Babel plugin that adds the pure annotation to output of TypeScript.

Pure annotations are very easy to miss and hard to get right, so perhaps we should automate this across the fp-ts ecosystem, using something like one of these tools. WDYT @gcanti?

@gcanti Have you tried to do anything like this before? If not then I can take a look.

[gcanti]

@OliverJAsh No I haven't, I was afraid to mess up the tsc output