thdesugar
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 handcoded 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 uptodate 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 thdesugar
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 thdesugar
'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, thdesugar
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, thdesugar
can only check for language extensions at use sites.
Limited support for kind inference
thdesugar
sometimes has to construct types for certain Haskell entities.
For instance, thdesugar
desugars all Haskell98style 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 thdesugar
performs over the type variable binders
of T
to extract all implicitly quantified variables and make them explicit.
This makes the desugared types forwardscompatible with a
future version of GHC
that requires all kind variables in a toplevel 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 thdesugar
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 thdesugar
will not be aware of it. Therefore, thdesugar
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 thdesugar
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:
 Desugared Haskell98style constructors
 Locally reified class methods
 Locally reified record selectors
 Locally reified data constructors
 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 thdesugar
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
thdesugar
supports writing patterns like (type a)
via the DTypeP
data
constructor of DPat
. Be warned, however, that thdesugar
only supports
desugaring DTypeP
in the clauses of function declarations, such as the
declaration of idv
above. As a result, thdesugar
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, thdesugar
supports desugaring the following:
idv' :: forall a > a > a
idv' a = id @a
But thdesugar
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 thdesugar
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
thdesugar
supports writing patterns like @a
via the DInvisP
data
constructor of DPat
. Be warned, however, that thdesugar
only supports
desugaring DInvisP
in the clauses of function declarations, such as the
declaration of f
above. As a result, thdesugar
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 thdesugar
'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.