th-desugar Package

This package provides the Language.Haskell.TH.Desugar module, which desugars Template Haskell's rich encoding of Haskell syntax into a simpler encoding. This desugaring discards surface syntax information (such as the use of infix operators) but retains the original meaning of the TH code. The intended use of this package is as a preprocessor for more advanced code manipulation tools. Note that the input to any of the ds... functions should be produced from a TH quote, using the syntax [| ... |]. If the input to these functions is a hand-coded TH syntax tree, the results may be unpredictable. In particular, it is likely that promoted datatypes will not work as expected.

One explicit goal of this package is to reduce the burden of supporting multiple GHC / TH versions. Thus, the desugared language is the same across all GHC versions, and any inconsistencies are handled internally.

The package was designed for use with the singletons package, so some design decisions are based on that use case, when more than one design choice was possible.

I will try to keep this package up-to-date with respect to changes in GHC.

Known limitations

th-desugar sometimes has to construct types for certain Haskell entities. For instance, th-desugar desugars all Haskell98-style constructors to use GADT syntax, so the following:

data T (a :: k) = MkT (Proxy a)

Will be desugared to something like this:

data T (a :: k) where
  MkT :: forall k (a :: k). Proxy a -> T (a :: k)

Notice that k is explicitly quantified in the type of MkT. This is due to an additional pass that th-desugar performs over the type variable binders of T to extract all implicitly quantified variables and make them explicit. This makes the desugared types forwards-compatible with a future version of GHC that requires all kind variables in a top-level forall to be explicitly quantified.

This process of extracting all implicitly quantified kind variables is not perfect, however. There are some obscure programs that will cause th-desugar to produce type variable binders that are ill scoped. Here is one example:

data P k (a :: k)
data Foo (a :: Proxy j) (b :: k) c = MkFoo c (P k j)

If you squint hard at MkFoo, you'll notice that j :: k. However, this relationship is not expressed syntactically, which means that th-desugar will not be aware of it. Therefore, th-desugar will desugar Foo to:

data Foo (a :: Proxy j) (b :: k) c where
  MkFoo :: forall j k (a :: Proxy j) (b :: k) c.
           c -> P k j -> Foo (a :: Proxy j) (b :: k) c

This is incorrect since k must come before j in order to be well scoped. There is a workaround to this issue, however: add more explicit kind information. If you had instead written this:

data Foo (a :: Proxy (j :: k)) (b :: k) c = MkFoo c (P k j)

Then the fact that j :: k is expressed directly in the AST, so th-desugar is able to pick up on it and pick forall k j (a :: Proxy j) (b :: k) c. <...> as the telescope for the type of MkFoo.

The following constructs are known to be susceptible to this issue:

1. Desugared Haskell98-style constructors
2. Locally reified class methods
3. Locally reified record selectors
4. Locally reified data constructors
5. Locally reified type family instances (on GHC 8.8 and later, in which the Template Haskell AST supports explicit foralls in type family equations)