New directions for Template Haskell

simonpj - 2010-10-18T21:55:06

Nota bene: TemplateHaskell/BlogPostChanges is a copy of this blog post, but with subsequent edits and improvements. Don’t pay too much attention to the text below; I’m keeping it only so that you can see the context for comments.

This post explores a set of design proposals for Template Haskell. They are inspired by discussion with Tim Sheard, Kathleen Fisher, and Jacques Carette. It was originally triggered by several Template Haskell tickets: including #4230, #4135, #4128, #4170, #4125, #4124, #4364, #6062, #6089. (See also #7016, which work better with the suggestions below.) Taken together, these proposals would make quite a big change to TH, I think for the better. Happily, I’m pretty sure they are relatively easy to implement.

There’s an interesting critique of Template Haskell on !StackOverflow, some (but not all) of which is addressed by proposals here.

But I’d like to get the design right; hence this post. I’m going to treat it as a working draft, and modify it in the light of comments. So please comment.

I’m going to assume that you are more or less familiar with Template Haskell. If not, there’s lots of background on the Template Haskell page. It’s a Wiki so you can help to improve it.

Here’s the implementation page the describes how we are going to implement this stuff.

Some brief background

After parsing, GHC runs two completely separate passes, one after the other: * The renamer resolves scopes, fixing precisely which binding site is connected which occurrence of every variable. For example, you write

and the renamer changes it to

The renamer also performs depdenency analysis, which sorts bindings (both value declarations and type declarations) into the smallest possible mutually recursive groups. This prior sorting is required to maximise polymorphism in mutually recursive value bindings.

  • The typechecker works on the renamed program, and typechecks it.

At the moment these two passes relate to Template Haskell as follows: * Quasi-quotes are run in the renamer. Why? Because quasi-quotes can expand to patterns. Consider this, which has a quasi-quoted pattern:

Is the “x” in “x+1” bound by the outer \\x or by the ‘x’ that might be bought into scope by the [pads| blah |] quasi-quote? The only way to know is to run the quasi-quote, so that’s what happens.

  • All other Template Haskell stuff is run in the typechecker. Why? Because we try to typecheck quotations before feeding them into TH functions. More on this below.

The main issue

The big issue is this: Template Haskell is both too weakly typed and too strongly typed.

Too weakly typed

A TH quotation has type Q Exp, a type that says nothing about the type of the quoted term. For example, consider

Presumably the author expects $x to be a boolean-valued term, but that is not checked. For example we might write

in which we pass a string-valued term to qnot. The splice will typecheck fine, but the returned code will be the ill-typed not "hello". There is no soundness problem because GHC typechecks the result of the splice $(qnot [| "hello" |]), but the error is reported in code that the user never wrote.

Moreover, errors can be delayed. For example, suppose qnot was like this:

This cannot possibly be right, becuase $x cannot be both a boolean and a list. Yet TH will accept it, because a splice has type forall a.a. The error will only be reported to callers of qnot.

This is bad. MetaML solves this problem by giving types to quoted terms, something like this:

Here TExp (short for typed expressions) has a type parameter that expresses the type of the quoted term.

In TH we deliberately did not do this, because it restricts expressiveness; see the original TH paper. We could make this choice without losing soundness because in TH, unlike in MetaML, all splicing is done at compile time, and the result of a splice is typechecked from scratch. But still, it’s a weakness and, for some users (stand up Jacques), a very serious weakness.

Too strongly typed

Even though TH cannot guarantee to construct only type-correct code, every quotation is typechecked. For example, the quotation [| "hello" && True |] would be rejected because it is internally inconsistent.

But with the advent of type splices (a very useful feature) typechecking quotes has become hard to do. Consider this:

This function f returns a declaration quote, declaring T and g. You’ll see that the constructor MkT takes an argument whose type is passed (as a quotation) to f – a type splice.

The difficulty is that we can’t typecheck the declaration of ‘g’ until we know what $t is; and we won’t know that until f is called. In short, * we can’t really typecheck the declaration quote at all An analogous problem occurs in other declaration splices, for example

Here GHC’s check that an instance decl is always of form

falls over, again because we don’t know what $t will be. Here’s another example:

We can’t sensibly typecheck the term without knowing what f’s type signature is, and we can’t know that without expanding the splice.

Here’s a rather different example, #4364:

Faced with a type splice

the renamer doesn’t know what the splice will expand to, because splices are currently run later, in the type checker. So it pessimistically assumes that the splice could expand to mention anything in scope. But that pessimistic assuption triggers the error message

     Cycle in type synonym declarations:
       m.hs:7:1-23: type N0 = $([t| Z |])
       m.hs:8:1-23: type N1 = $([t| Z |])  ```

All this is quite annoying. Several users have said “I’m just using a quotation as a convenient way to build a syntax tree. Please don’t even try to typecheck it; just wait until it is finally spliced into a top-level splice”.

# A proposal
TH currently embodies an uneasy compromise between being too strongly typed and too weakly typed. So my proposal is to move TH in both directions at once: * Part A: Move the existing structures towards the expressive but weakly-typed end. * Part B: Add new MetaML-style constructs for strongly-typed metaprogramming. * Part C: Clean up and improve reification * Part D: Quasi-quotation improvements

Part A: Reduce typechecking for quotes

On the “make-it-weaker” front, here’s what I propose:

  • Cease typechecking TH quotes altogether. Instead, to use GHC’s terminology, we would rename a quote, but not typecheck it. The renaming pass ensures that the scope hygiene mechanisms would remain unchanged. By not attempting to typecheck we avoid all the tricky problems sketched above.

  • Add pattern splices and local declaration splices, as requested in #1476. For example

These are not supported today because they bring new variables into scope, and hence are incompatible with running splices only after the renamer is finished; see Notes on Template Haskell, section 8.

  • Run TH splices in the renamer, uniformly with quasi-quotes. Of course, we must still typecheck the code we are about to run. But there’s an existing TH restriction that code run in top-level splices must be imported. So we can typecheck this code even during renaming, because it can only mention imported (and hence already fully-typechecked) code.

This solves #4364 because we run the splice in the renamer, so things are sorted out by the time we are checking for cycles (in the type checker).

  • Allow quoted names as patterns as requested by Conal Eliott. This is just a variation on allowing splices in patterns, since a quoted name 'x is really just a simple splice

These changes would essentially implement the desires of those who say “I just want to generate syntax trees”. All the mentioned bug reports would be fixed. The big loss is that quotes would not be typechecked at all.

Lexical scoping

Consider these definitions:

Where is the ‘y’ bound in the RHS of f?
* Perhaps by the y = 7 that is in scope at the definition of f? * Perhaps by the pattern that $(g n) expands to?
* Perhaps by a ‘y’ that is in scope at the splice site of f? * Does it depend on whether $(g n) in fact binds ‘y’? A major point about TH is that we get lexical scoping (also called “hygienic”). So, to me it seems the the first of these choices is the only reasonable one. If you want the second you can instead use explicit dynamic binding by saying

So the rule would be: * In a quote, a variable ‘v’ is bound by the lexically enclosing binding of ‘v’, ignoring all pattern and declaration splices. To be consistent this should apply to top level splices too.

A variation (probably not)

A possible, rather ad hoc, variation would be to still typecheck quotes that are (a) top level, and (b) expression quotes. For example, we might still reject this:

because the quote is obviously ill-typed. Only quotes nested inside top-level splices would avoid the type checker (because if the splice is run in the renamer, we can’t typecheck the nexted quote). For example:

This splice would run in the renamer, and only the result of the splice would be typechecked. But what about this?

This is still very awkward to typecheck. After all, if $t expands to a polymorphic type, the result of the splice might typecheck, but it’s really impossible to typecheck without knowing the signature. Maybe we should just give up if there’s a type splice? The only really simple thing is not to typecheck quotes at all.

Part B: Add MetaML-style typed quotes

Template Haskell has quotes for terms, types, patterns, and declarations. They are all untyped, in the sense that the type of the quote tells you nothing about the type of the quoted thing. For example

There’s no clue that the type of the quoted expression is Bool.

In the case of terms (only), we know how from MetaML to have typed quotations. Here’s a proposed extension to TH to add typed term quotes:

  • Add a new type of typed expressions TExp a

  • Add a new term quotation form [|| e ||], called a typed quote; the type of the quote is TExp ty, where ty is the type of e. In the type-system jargon, this is the “introduction form” for TExp.

  • Add a new splice form $$e, called a typed splice. The term e must have type TExp ty, and the splice $$e then has type ty. This is the “elimination form” for TExp.

  • Add a constant which takes a typed quote and returns an untyped one: unType :: TExp a -> Q Exp

  • Run these new typed splices in the typechecker, not the renamer.

(The precise syntax for typed-quotes and type-splices is of course up for grabs. But doubling the symbols seems plausible to me.)

Here’s a standard example:

You could do this with existing TH but there’d be no guarantee that power would return a well-typed term. With TExp there is.

Points to notice * Unlike TH, the only way to construct a value of type TExp is with a quote. You cannot drop into do-ntation, nor use explicit construction of the values in the Exp algebraic data type. That restriction limits expressiveness, but it enables the strong typing guarantees.

  • There are no declaration, type, or pattern quotes for these typed quotes. Only terms.

  • You can’t use an untyped splice in a typed quote, thus [|| ...$(e)... ||]. Similarly, you can’t splice a type, pattern, or declaration group in a typed term quote.

  • Using unType you can go from the typed world to the untyped one, which lets you mix the worlds. Example:

  • Unlike Exp, TExp is an abstract type, so you can’t decompose values of type TExp. All you can do with them is splice them (into a program or a larger quote). Or you can convert to a Q Exp and then decompose, using the existing TH mechanisms. For example
  • TExp is not a monad, but it is an applicative type constructor, although not quite an instance of Applicative class:

Reminder: the Applicative class looks like this

TExp is only “almost an instance” because pure isn’t a function; its argument must be syntactically quoted.

Syntax is always a delicate point. * We could use some other kind of bracket, although brackets are always in short supply; eg (| ... |) or {| ... |}. * We could add Unicode brackets too (suggestions?); but I think we should have ASCII equivalents. * Ian asked whether $(...) could accept either Q Exp or TExp. I think not; we need to know which kind of splice it is before type checking.

Part C: Reification and typechecking

The third part of this proposal concerns reification. The Q monad gives you access to the typechecker’s environment. Notably, Template Haskell provides the function

which, given the Name of a variable, type, or class, looks the Name up in the type environment and returns what the type checker knows about it:

What reify sees

A dark corner of reify is this: what types does reify see? Consider

Here, reify can be used to examine the type of x. But the type of x isn’t fully known until the type checker has seen the term (x && True). So in current TH it’s going to be unpredicatable what you see for x, which is hardly satisfactory.

It seems to me that the only decent, predictable story is to say that reify can only consult the top-level type environment. More specifically, Template Haskell processes a program top-down:

TH processes this module as follows: 1. Typecheck down to, but not including, the first splice, $(th1 4). These declarations constitute the first declaration group. 2. Typecheck and run the splice, which returns a bunch of declarations D1 3. Typecheck the declarations D1 combined with the declarations down to, but not including, the second splice. These declarations consitute the second declaration group. 4. Typecheck and run the next splice, $(th2 10) 5. Rinse and repeat

So the proposal is as follows. A declaration group is the chunk of declarations created by a top-level declaration splice, plus those following it, down to but not includig the next top-level declaration splice. Then the type environment seen by reify includes all the declaration up to the end of the immediately preceding declaration block, but no more.

So, in the preceding example: * A reify inside the splice $(th1 ..) would see the definition of f. * A reify inside the splice $(blah) woudl see the definition of f, but would not see any bindings created by $(th1...). * A reify inside the splice $(th2..) would see the definition of f, all the bindings created by $(th1..), and teh definition of h. * A reify inside the splice $(blah2) would see the same definitions as the splice $(th2...).

This would mean that you could not say

because there is no top=level splice between the declaration of f and the splice. But that seems reasonable to me. If you want that behaviour you can say

Reifying expressions

But what about expressions? It would be useful (stand up Kathleen) to have a more elaborate reify, like this:

(For GHCi users, reify f is like :info f, while typecheck [] (...) is like :type (...).)

You might ask whether we can typeckeck an expression; remember, these Q ty things are going to be run in the renamer. But if the type environment is that in force just before the last top-level splice, then all is well: that stuff has been fully typechecked.

Part D: quasiquotation

This part is unrelated to the preceding proposals, and is responding to #4372 and #2041.

  • For #2041, rather than the proposal made there, I think the nicest thing is for Language.Haskell.TH to expose a Haskell quasiquoter:

Remember that a QuasiQuoter is a quadruple of parsers:

If TH provided such parsers, you could use them to parse antiquotes. That seems better to than having strings in the TH syntax.

See #4430 for an excellent point about fixities.

  • For #4372, I’m a bit agnostic. There is no technical issue here; it’s just about syntax. Read the notes on the ticket.

  • See #4429 for a suggestion about reifying Names.

Part E: Other minor issues

This section collects other TH changes that I think should be done.

  • The InfixE construtor of Syntax.Exp should only allow a Var in the operator position. See Trac #4877