BSD-3-Clause licensed by Richard Eisenberg
Maintained by Ryan Scott

Module documentation for 1.10

This version can be pinned in stack with:th-desugar-1.10@sha256:cf7debc3f197c5d73f390e61a8be7b4853c8bf3c11e52e3da1338c3eb3af31d2,3464

th-desugar Package

Hackage Build Status

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)

Changes

th-desugar release notes

Version 1.10

  • Support GHC 8.8. Drop support for GHC 7.6.

  • Add support for visible kind application, type variable foralls in RULES, and explicit foralls in type family instances. Correspondingly,

    • There is now a DAppKindT constructor in DType.

    • Previously, the DDataInstD constructor had fields of type Name and [DType]. Those have been scrapped in favor of a single field of type DType, representing the application of the data family name (which was previously the Name) to its arguments (which was previously the [DType]).

      DDataInstD also has a new field of type Maybe [DTyVarBndr] to represent its explicitly quantified type variables (if present).

    • Previously, the DTySynEqn constructor had a field of type [DType]. That has been scrapped in favor of a field of type DType, representing the application of the type family name (which DTySynEqn did not used to contain!) to its arguments (which was previously the [DType]).

      DTySynEqn also has a new field of type Maybe [DTyVarBndr] to represent its explicitly quantified type variables (if present).

    • DTySynInstD no longer has a field of type Name, as that is redundant now that each DTySynEqn contains the same Name.

    • There is now a field of type Maybe [DTyVarBndr] in the DRuleP constructor to represent bound type variables in RULES (if present).

  • Add a field of type Maybe [DTyVarBndr] to DInstanceD and DStandaloneDerivD for optionally quantifying type variables explicitly. If supplied with a Just, this sweetens the instance type to use a ForallT to represent the explicit quantification. This trick is not supported for InstanceD on GHC 8.0 and for StandaloneDerivD on GHC 7.10 or 8.0, so be aware of this limitation if you supply Just for this field.

  • Add support for desugaring implicit params. This does not involve any changes to the th-desugar AST, as:

    • (?x :: a) => ... is desugared to IP "x" a => ....
    • id ?x is desugared to id (ip @"x").
    • let ?x = 42 in ... is desugared to let new_x_val = 42 in bindIP @"x" new_x_val ... (where bindIP is a new utility function exported by Language.Haskell.TH.Desugar on GHC 8.0 or later).

    In order to support this desugaring, the type signatures of dsLetDec and dsLetDecs now return ([DLetDec], DExp -> DExp) instead of just [DLetDec], where DExp -> DExp is the expression which binds the values of implicit params (e.g., \z -> bindIP @"x" new_x_val z) if any are bound. (If none are bound, this is simply the id function.)

  • Fix a bug in which toposortTyVarsOf would error at runtime if given types containing foralls as arguments.

  • Fix a bug in which fvDType would return incorrect results if given a type containing quantified constraints.

  • Fix a bug in which expandType would not expand type synonyms in the kinds of type variable binders in foralls.

  • Fix a bug in which getRecordSelectors would omit record selectors from GADT constructors.

  • Fix a bug in which toposortTyVarsOf would sometimes not preserve the left-to-right ordering of Names generated with qNewName.

  • Locally reified class methods, data constructors, and record selectors now quantify kind variables properly.

  • Desugared ADT constructors now quantify kind variables properly.

  • Remove DPred, as it has become too similar to DType. This also means that the DPat constructors, which previously ended with the suffix Pa, can now use the suffix P, mirroring TH.

  • The type of applyDType has changed from DType -> [DType] -> DType to DType -> [DTypeArg] -> DType, where DTypeArg is a new data type that encodes whether an argument is a normal type argument (e.g., the Int in Maybe Int) or a visible kind argument (e.g., the @Type in Proxy @Type Char). A TypeArg data type (which is like DTypeArg, but with Types/Kinds instead of DTypes/DKinds) is also provided.

    A handful of utility functions for manipulating TypeArgs and DTypeArgs are also exported.

  • th-desugar functions that compute free variables (e.g., fvDType) now return an OSet, a variant of Set that remembers the order in which elements were inserted. A consequence of this change is that it fixes a bug that causes free variables to be computed in different orders depending on which unique numbers GHC happened to generate internally.

  • Substition and type synonym expansion are now more efficient by avoiding the use of syb in inner loops.

Version 1.9

  • Suppose GHC 8.6.

  • Add support for DerivingVia. Correspondingly, there is now a DDerivStrategy data type.

  • Add support for QuantifiedConstraints. Correspondingly, there is now a DForallPr constructor in DPred to represent quantified constraint types.

  • Remove the DStarT constructor of DType in favor of DConT ''Type. Two utility functions have been added to Language.Haskell.TH.Desugar to ease this transition:

    • isTypeKindName: returns True if the argument Name is that of Type or (or *, to support older GHCs).
    • typeKindName: the name of Type (on GHC 8.0 or later) or * (on older GHCs).
  • th-desugar now desugars all data types to GADT syntax. The most significant API-facing changes resulting from this new design are:

    • The DDataD, DDataFamilyD, and DDataFamInstD constructors of DDec now have Maybe DKind fields that either have Just an explicit return kind (e.g., the k -> Type -> Type in data Foo :: k -> Type -> Type) or Nothing (if lacking an explicit return kind).

    • The DCon constructor previously had a field of type Maybe DType, since there was a possibility it could be a GADT (with an explicit return type) or non-GADT (without an explicit return type) constructor. Since all data types are desugared to GADTs now, this field has been changed to be simply a DType.

    • The type signature of dsCon was previously:

      dsCon :: DsMonad q => Con -> q [DCon]
      

      However, desugaring constructors now needs more information than before, since GADT constructors have richer type signatures. Accordingly, the type of dsCon is now:

      dsCon :: DsMonad q
            => [DTyVarBndr] -- ^ The universally quantified type variables
                            --   (used if desugaring a non-GADT constructor)
            -> DType        -- ^ The original data declaration's type
                            --   (used if desugaring a non-GADT constructor).
            -> Con -> q [DCon]
      

      The instance Desugar [Con] [DCon] has also been removed, as the previous implementation of desugar (concatMapM dsCon) no longer has enough information to work.

    Some other utility functions have also been added as part of this change:

    • A conExistentialTvbs function has been introduced to determine the existentially quantified type variables of a DCon. Note that this function is not 100% accurate—refer to the documentation for conExistentialTvbs for more information.

    • A mkExtraDKindBinders function has been introduced to turn a data type’s return kind into explicit, fresh type variable binders.

    • A toposortTyVarsOf function, which finds the free variables of a list of DTypes and returns them in a well scoped list that has been sorted in reverse topological order.

  • th-desugar now desugars partial pattern matches in do-notation and list/monad comprehensions to the appropriate invocation of fail. (Previously, these were incorrectly desugared into case expressions with incomplete patterns.)

  • Add a mkDLamEFromDPats function for constructing a DLamE expression using a list of DPat arguments and a DExp body.

  • Add an unravel function for decomposing a function type into its forall’d type variables, its context, its argument types, and its result type.

  • Export a substTyVarBndrs function from Language.Haskell.TH.Desugar.Subst, which substitutes over type variable binders in a capture-avoiding fashion.

  • getDataD, dataConNameToDataName, and dataConNameToCon from Language.Haskell.TH.Desugar.Reify now look up local declarations. As a result, the contexts in their type signatures have been strengthened from Quasi to DsMonad.

  • Export a dTyVarBndrToDType function which converts a DTyVarBndr to a DType, which preserves its kind.

  • Previously, th-desugar would silently accept illegal uses of record construction with fields that did not belong to the constructor, such as Identity { notAField = "wat" }. This is now an error.

Version 1.8

  • Support GHC 8.4.

  • substTy now properly substitutes into kind signatures.

  • Expose fvDType, which computes the free variables of a DType.

  • Incorporate a DDeclaredInfix field into DNormalC to indicate if it is a constructor that was declared infix.

  • Implement lookupValueNameWithLocals, lookupTypeNameWithLocals, mkDataNameWithLocals, and mkTypeNameWithLocals, counterparts to lookupValueName, lookupTypeName, mkDataName, and mkTypeName which have access to local Template Haskell declarations.

  • Implement reifyNameSpace to determine a Name’s NameSpace.

  • Export reifyFixityWithLocals from Language.Haskell.TH.Desugar.

  • Export matchTy (among other goodies) from new module Language.Haskell.TH.Subst. This function matches a type template against a target.

Version 1.7

  • Support for TH’s support for TypeApplications, thanks to @RyanGlScott.

  • Support for unboxed sums, thanks to @RyanGlScott.

  • Support for COMPLETE pragmas.

  • getRecordSelectors now requires a list of DCons as an argument. This makes it easier to return correct record selector bindings in the event that a record selector appears in multiple constructors. (See goldfirere/singletons#180 for an example of where the old behavior of getRecordSelectors went wrong.)

  • Better type family expansion (expanding an open type family with variables works now).

Version 1.6

  • Work with GHC 8, with thanks to @christiaanb for getting this change going. This means that several core datatypes have changed: partcularly, we now have DTypeFamilyHead and fixities are now reified separately from other things.

  • DKind is merged with DType.

  • Generic instances for everything.

Version 1.5.5

  • Fix issue #34. This means that desugaring (twice) is idempotent over expressions, after the second time. That is, if you desugar an expression, sweeten it, desugar again, sweeten again, and then desugar a third time, you get the same result as when you desugared the second time. (The extra round-trip is necessary there to make the output smaller in certain common cases.)

Version 1.5.4.1

  • Fix issue #32, concerning reification of classes with default methods.

Version 1.5.4

  • Added expandUnsoundly

Version 1.5.3

  • More DsMonad instances, thanks to David Fox.

Version 1.5.2

  • Sweeten kinds more, too.

Version 1.5.1

  • Thanks to David Fox (@ddssff), sweetening now tries to use more of TH’s Type constructors.

  • Also thanks to David Fox, depend usefully on the th-orphans package.

Version 1.5

  • There is now a facility to register a list of Dec that internal reification should use when necessary. This avoids the user needing to break up their definition across different top-level splices. See withLocalDeclarations. This has a side effect of changing the Quasi typeclass constraint on many functions to be the new DsMonad constraint. Happily, there are DsMonad instances for Q and IO, the two normal inhabitants of Quasi.

  • “Match flattening” is implemented! The functions scExp and scLetDec remove any nested pattern matches.

  • More is now exported from Language.Haskell.TH.Desugar for ease of use.

  • expand can now expand closed type families! It still requires that the type to expand contain no type variables.

  • Support for standalone-deriving and default signatures in GHC 7.10. This means that there are now two new constructors for DDec.

  • Support for static expressions, which are new in GHC 7.10.

Version 1.4.2

  • expand functions now consider open type families, as long as the type to be expanded has no free variables.

Version 1.4.1

  • Added Language.Haskell.TH.Desugar.Lift, which provides Lift instances for all of the th-desugar types, as well as several Template Haskell types.

  • Added applyDExp and applyDType as convenience functions.

Version 1.4.0

  • All Decs can now be desugared, to the new DDec type.

  • Sweetening Decs that do not exist in GHC 7.6.3- works on a “best effort” basis: closed type families are sweetened to open ones, and role annotations are dropped.

  • Infos can now be desugared. Desugaring takes into account GHC bug #8884, which meant that reifying poly-kinded type families in GHC 7.6.3- was subtly wrong.

  • There is a new function flattenDValD which takes a binding like let (a,b) = foo and breaks it apart into separate assignments for a and b.

  • There is a new Desugar class with methods desugar and sweeten. See the documentation in Language.Haskell.TH.Desugar.

  • Variable names that are distinct in desugared code are now guaranteed to have distinct answers to nameBase.

  • Added a new function getRecordSelectors that extracts types and definitions of record selectors from a datatype definition.

Version 1.3.1

  • Update cabal file to include testing files in sdist.

Version 1.3.0

  • Update to work with type Pred = Type in GHC 7.9. This changed the DPred type for all GHC versions, though.

Version 1.2.0

  • Generalized interface to allow any member of the Qausi class, instead of just Q.

Version 1.1.1

  • Made compatible with HEAD after change in role annotation syntax.

Version 1.1

  • Added module Language.Haskell.TH.Desugar.Expand, which allows for expansion of type synonyms in desugared types.
  • Added Show, Typeable, and Data instances to desugared types.
  • Fixed bug where an as-pattern in a let statement was scoped incorrectly.
  • Changed signature of dsPat to be more specific to as-patterns; this allowed for fixing the let scoping bug.
  • Created new functions dsPatOverExp and dsPatsOverExp to allow for easy desugaring of patterns.
  • Changed signature of dsLetDec to return a list of DLetDecs.
  • Added dsLetDecs for convenience. Now, instead of using mapM dsLetDec, you should use dsLetDecs.

Version 1.0

  • Initial release
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