# Towards Tactic Metaprogramming in Haskell

Isn’t it weird that we treat source code as text? That is, we have this extremely well-structured and strongly-typed object — the abstract syntax tree — that exists conceptually in our minds, and in actuality inside of our compiler, but for some reason we pretend it’s just a pile of bytes and edit it byte-by-byte rather than semantically?

When you stop and think about it, that’s like the stupidest idea ever. We as the authors don’t think of our code as bytes, nor does our interpreter or compiler. But instead we take the semantic understanding inside of our heads, serialize it into bytes, and then get the compiler to parse and rediscover the ideas inside our head. What a waste of effort.

Instead, you can use the incredible TOTBWF and my new Tactics Plugin for the Haskell Language Server, which will automatically and intelligently fill holes in your Haskell programs.

This blog post describes what a tactics engine is and why you want one, and is a good introduction to how in the hell we can automatically write your code for you.

## Tactics

Imagine you’re pair programming with a junior engineer. In the navigator seat, you’ll be guiding your partner through the implementation, guiding them through the high-level strokes and letting them actually do the coding part. In order to implement foldr :: (a -> b -> b) -> b -> [a] -> b, for example, the guidance you give your partner might be:

1. Bind the function arguments
2. Case split on the [a] parameter
3. If it’s [], do the obvious thing
4. Otherwise call your function and recurse.

These instructions aren’t a real program by any means, but you might call them a “program sketch.” The hard part of programming (thinking about what to do) is captured here, but actually doing it is left as an exercise to the reader.

A tactics engine transforms a program sketch like the above into an actual program. Tactics free us from the tyranny of text editing and pedantic details, allowing us to work at a higher semantic level of programming.

Tactics correspond to semantic operations over our program. Much like how the primitive commands in text editors (delete to end of line, insert parentheses, etc) can be composed to refine the textual representation of one program into the textual representation of another, we can compose small tactics in order to build larger ideas.

As an example, consider how we can fill in the following hole:

data Id a = Id a

instance Functor Id where
fmap :: (a -> b) -> Id a -> Id b
fmap = _

Rather than writing this function all at once, we can instead build it, one idea at a time. The first step is obviously to bind function arguments (the intros tactic), which results in the refined expression:

fmap :: (a -> b) -> Id a -> Id b
fmap = \fab ida -> _

We’re left with a new hole, but this one is “smaller” than the old one; we’ve refined the previous hole, filling in some of its structure. As a result, the type of our new hole is Id b, and we now have both fab :: a -> b and ida :: Id a in scope. We can simplify the hole further by now pattern matching on ida (the destruct ida tactic):

fmap :: (a -> b) -> Id a -> Id b
fmap = \fab ida -> case ida of
Id a -> _

The resulting hole still has type Id b, but we’ve now introduced a :: a in scope. Our next step is to build an Id value, which we can do by producing its data constructor (the split tactic):

fmap :: (a -> b) -> Id a -> Id b
fmap = \fab ida -> case ida of
Id a -> Id _

Again we’ve shrunk the problem — now our hole has type b. At this point we can call the fab function to produce a b (via the apply fab tactic):

fmap :: (a -> b) -> Id a -> Id b
fmap = \fab ida -> case ida of
Id a -> Id (fab _)

All that’s left is a hole with type a. Fortunately, we have a :: a in scope, so we can just plop that in to the hole via the assumption tactic:

fmap :: (a -> b) -> Id a -> Id b
fmap = \fab ida -> case ida of
Id a -> Id (fab a)

And just like that, we’ve produced an implementation of our desired function! By thinking in terms of the semantic operations we’d like to perform at each hole (instead of how to manipulate the bytes of text), we’ve changed the level of abstraction at which we think about editing. The implications of this might not be immediately obvious, so let’s explore them together.

Let’s list the tactic steps we took to derive fmap:

intros
destruct ida
split
apply fab
assumption

Up to alpha renaming, this composition of tactics is sufficient to derive fmap for any sum or product type that doesn’t do anything “exciting” with its type variable. By running the same steps, we can implement fmap for any of the following types:

(a, b)
Either a b
Maybe a
Const a b

Let’s convince ourselves of this by quickly running through the derivation for Maybe a. We start again with fmap and its type:

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap = _

After running intros:

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap = \fab ma -> _

and then destruct ma

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap = \fab ma -> case ma of
Nothing -> _
Just a  -> _

Applying split here is a little tricky; technically it will force us to try both Nothing and Just _ at each hole in a weird sort of quantum superposition. Let’s ignore this detail for right now, and come back to it immediately after finishing the derivation. Assuming we pick the right data cons, after split our program looks like this:

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap = \fab ma -> case ma of
Nothing -> Nothing
Just a  -> Just _

Now we run apply fab. Because Nothing doesn’t take any arguments, it didn’t produce any holes, so we need look only at the Just case:

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap = \fab ma -> case ma of
Nothing -> Nothing
Just a  -> Just (fab _)

and finally we run assumption to fill in the hole:

fmap :: (a -> b) -> Maybe a -> Maybe b
fmap = \fab ma -> case ma of
Nothing -> Nothing
Just a  -> Just (fab a)

Look at that! Even though it would require significantly different editing commands to write the syntax of these two functor instances, they are both descried by the same composition of tactics. This is what I mean by “semantic editing,” we’re moving the algorithm for producing functor instances out of our heads and reifying it into something the computer understands. In essence, by writing fmap once, we can teach the computer how to write it for us in the future.

I mentioned earlier that split gives us some issues here. Reading closely, you’ll notice that there is nothing in our tactics that say we need to split the same data constructor that we just destructed. In actuality there are four different, valid programs that can be produced by the above set of tactics:

fmap = \fab ma -> case ma of
Nothing -> Nothing
Just a  -> Nothing

fmap = \fab ma -> case ma of
Nothing -> Nothing
Just a  -> Just (fab a)

fmap = \fab ma -> case ma of
Nothing -> Just _
Just a  -> Nothing

fmap = \fab ma -> case ma of
Nothing -> Just _
Just a  -> Just (fab a)

Choosing the “best” implementation of these possibilities is largely a matter of heuristics, which I plan to describe in a later post. For now, let’s just assume our tactics engine is smart enough to come up with the one you had in mind.

Of course, the real issue here is that nothing forces our destruct and split tactics to use the same data constructor. We can eliminate this ambiguity by noticing that in fmap, we’re not actually trying to destruct and then split, but instead we’re trying to implement a homomorphism (a structure-preserving function.) In order to preserve structure, we’d better map a data constructor to itself. So instead, let’s use the homo tactic instead of destruct and split. Our new tactics metaprogram for writing functor instances is thus:

intros
homo ida
apply fab
assumption

This new version now can no longer generate any of the pathological fmap implementations for Maybe, as they are not structure preserving. We’re left only with the good implementation. Let’s do one more derivation, this time for Either c a. After intros and homo eca, we’re left with:

fmap :: (a -> b) -> Either c a -> Either c b
fmap = \fab ma -> case eca of
Left  c -> Left _
Right a -> Right _

For the first time, we’re now left with two holes. The default behavior is for a tactic to apply to all holes (although there are combinators for “zipping” holes), meaning that the apply fab tactic will be run on both holes. For the Left case, our hole has type c, but fab _ has type b, so this tactic fails to apply here. Tactic failure is per-hole, so we can still apply it to the other hole, resulting in:

fmap :: (a -> b) -> Either c a -> Either c b
fmap = \fab ma -> case eca of
Left  c -> Left _
Right a -> Right (fab _)

And finally, assumption fills the hole with whatever would typecheck. In the first hole that’s c, and in the second it’s a as before.

fmap :: (a -> b) -> Either c a -> Either c b
fmap = \fab ma -> case eca of
Left  c -> Left c
Right a -> Right (fab a)

Amazing! Three different functor implementations, with different numbers of data constructors, type variables, and cardinalities. By programming at the level of tactics rather than bytes, we can ignore the superficial differences between these implementations, focusing instead on the fact that they’re all derived the same way.

Hopefully this post has given you some insight into what tactics are and why they’re valuable. In the next post we’ll look at how this stuff is implemented behind the scenes, and the difficulties we’ve had integrating it into the language server. Stay tuned!