rich kinds

Changes to Template Haskell Library

Throughout this document, it is assumed that the current version of GHC is 7.4.1.

A proposed change will add the following constructors to TH's Type datatype...

| PromotedListT [Type]    -- for types of the form '[Int, Bool]
| PromotedTupleT [Type]   -- for types of the form '(Int, 'False)
| PromotedConsT           -- ':

... and the following constructors to TH's Kind datatype:

| ConK Name               -- for kinds of the form Bool
| VarK Name               -- k
| AppK Kind Kind          -- k1 k2
| ListK                   -- []
| TupleK Int              -- (), (,), ...
| ConstraintK             -- Constraint

Note that there is no ForallK constructor because the internal GHC representation for kinds with variables does not use this. Kinds are automatically generalized over an entire type expression.

TH will also need to support promoted constructors other than lists and tuples, but this is in fact already supported through the use of ConT. The namespace of defined types and of promoted types is also already kept distinct. For example, if we have the definition data Foo = Foo, the results of [t| Foo |] and [t| 'Foo |] are distinct (as in, == returns False). However, applying show to these two results produces the same string. Using the naming quote syntax, we can access promoted data constructors using the single-quote form. For example, ConT 'False denotes the promoted data constructor 'False. (Note that the parsed interpretations of the ' in these two snippets are entirely unrelated.)

Alternatives

Here are two alternatives to the above changes. They are orthogonal to each other (i.e. either can be chosen without affecting whether or not the other is chosen).

Simpler Types

Instead of the new types above, we could have the following:

| PromotedTupleT Int     -- '(), '(,), ...
| PromotedNilT           -- '[]
| PromotedConsT          -- ':

Client code could create full tuples and lists using a combination of the above constructors with a liberal sprinkling of AppTs.

• Pros: Matches syntax of existing TupleT and ListT. For lists, matches forms available in surface syntax.
• Cons: Believed to be harder to use in practice. The PromotedTupleT construct here is not available in surface syntax. This also loses the ability to write succinct lists, while the original format proposed above allows for nil as a 0-element list.

It may be worth noting that '(,) Int Bool is not a synonym for '(Int,Bool). '(,) Int Bool is a parse error.

Structured Kinds

Instead of the new ListK and TupleK kinds above, we could have the following:

| ListK Kind     -- [k]
| TupleK [Kind]  -- (k1,k2,...)

• Pros: Perhaps easier to use. Mirrors surface syntax.
• Cons: Does not match internal GHC representation, including what is printed in error messages and such. Different from the way Type works.

It may be worth noting that, in the kind language, (,) Int Bool is not a synonym for (Int,Bool). (,) Int Bool is a parse error.