Dhall.Map: Forget order of keys even harder

[sjakobi]

So far, this speeds up the "issue 412" benchmark by ~9% and "union performance"
by ~2%.

I expect further speedups when we improve the Map usage in e.g. Dhall.Eval. We could also try ignoring the original key order right from the beginning for the commands where we don't care about it, like normalization.

55e222b is cherrypicked from #1049.

[EggBaconAndSpam]

Wouldn't it make more sense to wait for the NbE-elaboration stuff before working on micro-optimisations? See @ocharles's benchmark results (here); he gets a multiple-order-of-magnitude speedup.

[Gabriella439]

@EggBaconAndSpam: it is definitely still worth doing, for a few reasons:

• Doing so will make it easier to identify and fix other performance bottlenecks that we might not be aware of yet

• We will probably need to do this anyway even after we merge the much faster type checker. Users have a tendency to push the language to it's performance limits regardless of how fast we make things

• Issue 412 is an example of an expression that might not see order-of-magnitude improvement from the faster type-checking branch (it might only be 4x, which is good but still needs improvement)

• 9% is actually quite a good improvement for such a small change

[sjakobi]

Are these all the dhall commands that need to preserve ordering of fields and constructors?

• resolve
• diff
• lint
• format
• freeze
• decode

That would leave the following commands that could forget about the ordering early:

• type
• normalize
• repl
• hash
• encode

How early? I'm not sure whether to try messing with the parser to control Dhall.Parser.Combinators.toMap. But already the import resolution might be slightly quicker if doesn't have to traverse maps in their original order.

I guess I don't have a good idea of dhalls overall structure. Do you have an idea where a "forget-key-order" step could fit in @Gabriel439?

[Gabriella439]

This looks good to me as is. I'm not sure if you still have any additional changes planned for this pull request

[Gabriella439]

@sjakobi: Sorry, I missed your earlier question. Only dhall {lint,format,freeze} need to preserve field order. Everything else can freely reorder fields if it's more efficient that way

[sjakobi]

@Gabriel439 Thanks, that's good to know! I'll probably try to hook into Dhall.Parser.Combinators.toMap so it doesn't produce the keys list for the other commands.