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. The minimum supported version of GHC is 8.0, which was chosen to avoid various Template Haskell bugs in older GHC versions that affect how this library desugars code. If this choice negatively impacts you, please submit a bug report.

Known limitations

Desugaring depends on language extensions of use sites

Suppose you quote some Template Haskell declarations in module A:

{-# LANGUAGE ... #-}
module A where

decs :: Q [Dec]
decs = [d| ... |]

And later desugar the declarations with th-desugar in module B:

{-# LANGUAGE ... #-}
module B where

import A (decs)
import Language.Haskell.TH.Desugar (dsDecs)

$(do desugaredDecs <- dsDecs decs
     ...)

There are some situations where th-desugar's desugaring depends on which language extensions are enabled, such as:

• MonadFailDesugaring (for desugaring partial pattern matches in do notation)
• NoFieldSelectors (for determining if a record field can be reified as a field selector with lookupValueNameWithLocals)

Somewhat counterintuitively, th-desugar will consult the language extensions in module B (the site where the decs are used) for this process, not module A (where the decs were defined). This is really a Template Haskell limitation, since Template Haskell does not offer any way to reify which language extensions were enabled at the time the declarations were defined. As a result, th-desugar can only check for language extensions at use sites.

Limited support for kind inference

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)

Limited support for linear types

Currently, the th-desugar AST deliberately makes it impossible to represent linear types, and desugaring a linear function arrow will simply turn into a normal function arrow (->). This choice is partly motivated by issues in the way that linear types interact with Template Haskell, which sometimes make it impossible to tell whether a reified function type is linear or not. See, for instance, GHC#18378.

Limited support for embedded types in patterns

In GHC 9.10 or later, the RequiredTypeArguments language extension allows one to write definitions with embedded types in patterns, e.g.,

idv :: forall a -> a -> a
idv (type a) = id @a

th-desugar supports writing patterns like (type a) via the DTypeP data constructor of DPat. Be warned, however, that th-desugar only supports desugaring DTypeP in the clauses of function declarations, such as the declaration of idv above. As a result, th-desugar does not support desugaring DTypeP in any other position, including:

• Lambda expressions. For example, the following is not supported:

  idv2 :: forall a -> a -> a
  idv2 = \(type a) -> id @a
  

• \case expressions. For example, the following is not supported:

  idv3 :: forall a -> a -> a
  idv3 = \case
    (type a) -> id @a
  

• \cases expressions. For example, the following is not supported:

  idv4 :: forall a -> a -> a
  idv4 = \cases
    (type a) x -> x :: a
  

Note that all of the example above use an explicit type keyword, but the same considerations apply for embedded type patterns that do not use the type keyword. That is, th-desugar supports desugaring the following:

idv' :: forall a -> a -> a
idv' a = id @a

But th-desugar does not support desugaring any of the following:

idv2' :: forall a -> a -> a
idv2' = \a -> id @a

idv3' :: forall a -> a -> a
idv3' = \case
  a -> id @a

idv4' :: forall a -> a -> a
idv4' = \cases
  a x -> x :: a

As a workaround, one can convert uses of lambdas and LambdaCase to function declarations, which are fully supported. See also this th-desugar issue, which proposes a different approach to desugaring that would allow all of the examples above to be accepted.

Limited support for invisible type patterns

In GHC 9.10 or later, the TypeAbstractions language extension allows one to write definitions with invisible type patterns, e.g.,

f :: a -> a
f @a = id @a

th-desugar supports writing patterns like @a via the DInvisP data constructor of DPat. Be warned, however, that th-desugar only supports desugaring DInvisP in the clauses of function declarations, such as the declaration of f above. As a result, th-desugar does not support desugaring DInvisP in any other position, such as lambda expressions or \cases expressions.

Ultimately, this limitation has the same underlying cause as th-desugar's limitations surrounding embedded types in patterns (see the "Limited support for embedded types in patterns" section above). As a result, the same workaround applies: convert uses of lambdas and LambdaCase to function declarations, which are fully supported.