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Note that these are tactics for metaprogramming - the tactics are themselves for programming, not for use in programs. The commands to the LSP tell it how to generate the code, and the code sticks around for editing afterwards.

>If it's just to be used by the LSP: What is the point of using tactics vs normal metaprogramming?

What is "normal" metaprogramming? The particular benefit of using tactics would be that they're designed for small steps of in-flight editing, rather than generating something all at once; and that their vocabulary is pretty well established.

>Note that these are tactics for metaprogramming - the tactics are themselves for programming, not for use in programs.

I don't understand what this is supposed to mean. How are e.g. coq tactics not the exact same?

>The commands to the LSP tell it how to generate the code, and the code sticks around for editing afterwards.

Not having the metaprogram which generated the code around just strikes me more as an anti-feature.

>What is "normal" metaprogramming?

Arbitrary, "non-sequential" code transformations.

>The particular benefit of using tactics would be that they're designed for small steps of in-flight editing, rather than generating something all at once

Well, for me tactics definitely invokes more serious proof automation than just what you could also do using Agda's emacs commands. This is also the impression I get from the article, the image presented uses some form of auto tactic to generate an entire term in one go. Certainly not what I'd call "small steps of in-flight editing".

> I don't understand what this is supposed to mean. How are e.g. coq tactics not the exact same?

Well, I've never gotten `induction' to generate anything close to a readable program -- hopefully this is better?

I'm not sure what you mean? The induction tactic uses the induction principle (dependent eliminator) of the corresponding type. If you unfold the dependent eliminator, it boils down to a combination of fix and match. I don't see how you could get a term simpler than that, unless you mean something else by readable?

IIRC, the match it generates often ends up being a dependent match using the convoy pattern once nontrivial functions start being written, which quickly gets really really hard to read. Maybe this is partly the fault of Coq's matching requiring the convoy pattern (unlike Agda's or Idris's), but I find it extremely difficult to read these terms, versus hand-written ones.

>I don't understand what this is supposed to mean. How are e.g. coq tactics not the exact same?

Coq tactics are code - you write terms using tactics, and you edit the term by editing the tactics. Save and load the file, and only the tactics stick around.

In this version, tactics write code - you use tactics to generate a term (Haskell code), and then you can edit the term itself. Save and load the file, and all you get is the Haskell code: the tactics only existed for codegen.

>Not having the metaprogram which generated the code around just strikes me more as an anti-feature.

Why? The trouble with metaprogramming is that you basically never want to have the power to simultaneously edit the program and the metaprogram. As you yourself pointed out, tactics would be terrible as an opaque metaprogram - they only work if you don't care about the particular term they generate.

Tactic metaprogramming lets you write terms quickly - it fulfills a need that lots of Haskell programmers have, which is that certain type signatures have "obvious" inhabitants and actually expressing those inhabitants can itself feel like a lot of boilerplate. But since you need to edit the Haskell code itself, the metaprogram must disappear. You won't miss it, since you won't need to regenerate the program from the metaprogram.

>the image presented uses some form of auto tactic to generate an entire term in one go

Because it's built out of several tactic steps, which the article goes into the construction of. You could still (presumably) have built it piece by piece, by applying each tactic one at a time - and each step only modifies the term slightly, leaving a hole for later steps.

>In this version, tactics write code - you use tactics to generate a term (Haskell code), and then you can edit the term itself. Save and load the file, and all you get is the Haskell code: the tactics only existed for codegen.

There's nothing preventing you from already doing this. You can use Isabelle's hammer/CoqHammer to generate a proof, dump the proof term and then replace your usage of the hammer tactic with the term. It's just, why would you want to edit the proof term manually? I'm pretty sure you can do this with Haskell's normal metaprogramming facilities too, it's just really not the intended way.

>Why?

Because it's a lot harder to maintain. Not only do you lose out on information about the generation (Why/How was it generated?), if you change the code it was generated from, you'll have to either manually edit the generated code (especially troublesome because the information of how it was generated was thrown away!) or you'll have to figure out how to do it using tactics again (in that case, why not just keep the tactics as documentation?).

>As you yourself pointed out, tactics would be terrible as an opaque metaprogram - they only work if you don't care about the particular term they generate.

Tactics aren't 'opaque', it's just a pain to manually look at the generated code and I don't see how the same doesn't apply here.

>Because it's built out of several tactic steps, which the article goes into the construction of. You could still (presumably) have built it piece by piece, by applying each tactic one at a time - and each step only modifies the term slightly, leaving a hole for later steps.

You could, but it seems pretty clear to me that this is not the intention. If you just want some form of 'quick actions' you don't need a tactics system. Having an option to 'try filling this hole' heavily indicates that this is to be used for more heavy automation.

>It's just, why would you want to edit the proof term manually?

Because you want to produce a particular proof term, because you are programming, not proving. Even with the CHC, there is a distinction between the two.

>Not only do you lose out on information about the generation (Why/How was it generated?), if you change the code it was generated from, you'll have to either manually edit the generated code (especially troublesome because the information of how it was generated was thrown away!)

This is a standard part of programming. Very rarely does editing code come with the complement of how/why the code itself was written - and even it does (like through doc comments) editing the code can then invalidate that information because the motivation can become outdated.

>Tactics aren't 'opaque', it's just a pain to manually look at the generated code and I don't see how the same doesn't apply here.

Because you want to look at the code, because the generated code is the objective of writing the tactics. You are not programming with tactics, you are writing code with tactics.

The point is this: if you wanted to prove `a -> a -> a`, then `intros; assumption` is a fine proof. But if you want to define `min :: (Ord a) => a -> a -> a`, then `intros; assumption` would typecheck without doing what you want. Tactics are not a good fit for programming, in general, because they fit the very different purpose of finding inhabitants rather than specifying them.

By contrast, actually writing code does seem like a good fit for tactics, because the steps you might take to iteratively program the particular term you want fit nicely with individual tactics.

For the recursive functor application example on a tree, normal meta-programming would be a macro taking the type as an argument, and then generating the resulting case by case application code, at compile time.

This would be much clearer and could automatically keep working if more cases were added (adding new node types is common when adding language features to abstract syntax trees).

This is a very awesome demo but my experience with other templating systems in IDEs (like autogenerated toString and other methods in Java) is that making boilerplate easy to write doesn't help make it easier to read or modify.

I agree; the most exciting use case for a system like this would be that it would functionally (no pun intended) replace the idea of "tab-completion" for typed functional programming languages.

> I hate to be a downer, but I just don't trust automatic code generation.

Isn't all programming in high level languages really just automatic code generation?

https://en.wikipedia.org/wiki/Autocode

> Also, it isn't clear to me whether tactics programs are supposed to be kept as code or just used by the LSP which the gif makes it seem like.

I'm pretty sure you use it as a starting point and then go from there. Saves a good amount of mechanical work.