Optimising at the AST level

[Rich-Harris]

Second random thought after #645. In cases like this...

{#if foo}
  <p>foo!</p>
{:else}
  <p>not foo!</p>
{/if}

...we have to create three blocks (create_main_fragment, create_if_block and create_if_block_1), totalling 74 LOC as of the current version. If we optimised it to the equivalent of this...

<p>{foo ? 'foo!' : 'not foo!'}</p>

...we get the same result with one block (create_main_fragment) totalling 25 LOC, and there's less work to do to update things.

There are probably all sorts of optimisations like this we could perform. Might not be straightforward, but definitely worth investigating.

[mrkishi]

On the if optimization, we could actually get better output than the equivalent {foo ? 'foo!' : 'not foo!'}, since the ternary ends up duplicated as-is:

function create_main_fragment(component, ctx) {
	var p, text_value = ctx.foo ? 'Some large static string...' : 'Or an even bigger one here!', text, current;
	return {

		[...]

		p: function update(changed, ctx) {
			if ((changed.foo) && text_value !== (text_value = ctx.foo ? 'Some large static string...' : 'Or an even bigger one here!')) {
				setData(text, text_value);
			}
		}

		[...]

	};
}

[Rich-Harris]

Yeah, that's unfortunate. I've wondered about having block-level helpers like

function create_main_fragment(component, ctx) {
  var p, get_text = ctx => ctx.foo ? 'Some large static string...' : 'Or an even bigger one here!', text_value=get_text(ctx), text, current;
  return {
    // ...
    p(changed, ctx) {
      if (changed.foo && (text_value !== get_text(ctx)) {
        setData(text, text_value);
      }
    }
  };
}

In fact those could be hoisted, and potentially even deduplicated across a bundle:

const get_text = ctx => ctx.foo ? 'Some large static string...' : 'Or an even bigger one here!';

function create_main_fragment(component, ctx) {
  var p, text_value=get_text(ctx), text, current;
  return {
    // ...
    p(changed, ctx) {
      if (changed.foo && (text_value !== get_text(ctx)) {
        setData(text, text_value);
      }
    }
  };
}