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.
Changes
th-desugar release notes
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.
