
GHC 8.10.x Migration Guide

Compiler changes
 Implicit kind variable changes
 GHC infers fewer dependent kinds
 GHC is pickier about impredicative uses of constraints
 GHC is pickier about inferred type signatures
 TypeFamilyDependencies may require UndecidableInstances
 New RecordWildCards warnings
 Improvements to pattern match checking with userdefined COMPLETE sets
 Library changes

Compiler changes
GHC 8.10.x Migration Guide
This guide summarises the changes you may need to make to your code to migrate from GHC 8.8 to GHC 8.10. This guide complements the GHC 8.10.x release notes which should be consulted as well.
Compiler changes
Implicit kind variable changes
GHC 8.10 implements proposal 103, which means that GHC is much less likely to implicitly quantify kind variables than it used to be. Here are some examples of code which will no longer work with GHC 8.10:

Kind variables are no longer implicitly quantified when an explicit
forall
is used at the beginning of a function's type signature. For instance, the following will no longer work:{# LANGUAGE PolyKinds #} {# LANGUAGE ScopedTypeVariables #} f :: forall (a :: k). Proxy a f = Proxy
error: Not in scope: type variable ‘k’  6  f :: forall (a :: k). Proxy a  ^
This is because
k
is implicitly quantified in the kind ofa
. Here are two potential ways to migrate this code:
If you are using GHC 8.0 or later, you can simply quantify
k
explicitly:f :: forall k (a :: k). Proxy a f = Proxy
Note that GHC 8.0, 8.2, and 8.4 require enabling the
TypeInType
extension in order to do this. On GHC 8.6 or later, however, explicitly quantifying kind variables simply requires thePolyKinds
extension. 
If you need to support versions of GHC older than 8.0, you may find the following piece of
CPP
useful:#if __GLASGOW_HASKELL__ >= 800 # define KVS(kvs) kvs #else # define KVS(kvs) #endif f :: forall KVS(k) (a :: k). Proxy a f = Proxy


Kind variables are no longer implicitly quantified in data constructor declarations:
data T a = T1 (S (a :: k))  forall (b::k). T2 (S b)  no longer accepted data T (a :: k) = T1 (S (a :: k))  forall (b::k). T2 (S b)  still accepted
As the above examples show, code that breaks because of this change can generally be fixed by adding explicit kind signatures to the type variable binders of the data type itself.

Implicitly quantified kind variables are no longer put in front of other variables:
f :: Proxy (a :: k) > Proxy (b :: j)
ghci> :t +v f  old order: f :: forall k j (a :: k) (b :: j). Proxy a > Proxy b ghci> :t +v f  new order: f :: forall k (a :: k) j (b :: j). Proxy a > Proxy b
This is a breaking change for users of
TypeApplications
. If you wish to restore the old order, then explicitly quantify the type variables off
. 
In type synonyms and type family equations, free variables on the righthand side are no longer implicitly quantified unless used in an outermost kind annotation:
type T = Just (Nothing :: Maybe a)  no longer accepted type T = Just Nothing :: Maybe (Maybe a)  still accepted
GHC infers fewer dependent kinds
GHC 8.10 features improvements to its kind inference engine. One downside to this is that it is slightly more conservative about inferring dependent kinds than it used to be. For example, the following example will kindcheck on old versions of GHC, but not with GHC 8.10:
{# LANGUAGE DataKinds, GADTs, PolyKinds, ScopedTypeVariables #}
module Foo where
import Data.Kind
data T a where
MkT1 :: T Int
MkT2 :: T Bool
data ST a :: T a > Type where
SMkT1 :: ST Int MkT1
SMkT2 :: ST Bool MkT2
Foo.hs:11:20: error:
• Expected kind ‘T a’, but ‘MkT1’ has kind ‘T Int’
• In the second argument of ‘ST’, namely ‘MkT1’
In the type ‘ST Int MkT1’
In the definition of data constructor ‘SMkT1’

11  SMkT1 :: ST Int MkT1
 ^^^^
To fix this issue in a backwardscompatible way, one can give ST
a complete, userspecified kind signature (consult the relevant users' guide section for a more detailed description on what this means). In other words, apply the following change:
data ST a :: T a > Type where
+data ST (a :: Type) :: T a > Type where
GHC is pickier about impredicative uses of constraints
Now that issue #11514 (closed) has been fixed, GHC is more stringent about impredicative uses of constraints. For example, the following program would be accepted on older versions of GHC, but not with GHC 8.10 or later:
foo :: forall a. (Show a => a > a) > ()
foo = undefined
foo
is impredicative since it attempts to instantiate undefined
at type (Show a => a > a) > ()
. This can be fixed by simply adding the necessary wildcard patterns, like so:
foo :: forall a. (Show a => a > a) > ()
foo _ = undefined
GHC is pickier about inferred type signatures
GHC now performs more validity checks on inferred type signatures. One consequence of this change is that some programs that used to be accepted will no longer compile without enabling the required language extensions. For example, in these two modules:
{# LANGUAGE RankNTypes #}
module A where
foo :: (forall a. a > a) > b > b
foo f x = f x
module B where
import A
bar = foo
Notice that A
enables RankNTypes
, but B
does not. Previous versions of GHC would allow bar
to typecheck, even though its inferred type is higherrank. GHC 8.10 will now reject this, as one must now enable RankNTypes
in B
to accept the inferred type signature.
TypeFamilyDependencies
may require UndecidableInstances
Type family dependencies (also known as injective type families) sometimes now need UndecidableInstances
in order to be accepted. Here is an example:
type family F1 a = r  r > a
type family F2 a = r  r > a
type instance F2 [a] = Maybe (F1 a)
Because GHC needs to look under a type family to see that a
is determined by the righthand side of F2
's equation, this now needs UndecidableInstances
. The problem is very much akin to its need to detect some functional dependencies.
RecordWildCards
warnings
New GHC 8.10 introduces two new flags, Wunusedrecordwildcards
and Wredundantrecordwildcards
, which are implied by Wall
. This means that certain programs that use RecordWildCards
will trigger warnings that did not do so previously. For instance, the following program emits no warnings on old versions of GHC, but will warn with GHC 8.10:
{# LANGUAGE NamedFieldPuns #}
{# LANGUAGE RecordWildCards #}
{# OPTIONS_GHC Wall #}
data P = P { x :: Int, y :: Int }
f1 :: P > Int
f1 P{..} = 1 + 3
f2 :: P > Int
f2 P{x,y,..} = x + y
Foo.hs:8:6: warning: [Wunusedrecordwildcards]
No variables bound in the record wildcard match are used
Possible fix: omit the ‘..’

8  f1 P{..} = 1 + 3
 ^^
Foo.hs:11:10: warning: [Wredundantrecordwildcards]
Record wildcard does not bind any new variables
Possible fix: omit the ‘..’

11  f2 P{x,y,..} = x + y
 ^^
To fix the warnings, simply remove the uses of ..
, as neither use is necessary.
Improvements to pattern match checking with userdefined COMPLETE sets
Previously, the user guide specified a couple of heuristics to define how pattern match warnings with userdefined COMPLETE sets are handled. Now these heuristics (and surprising behavior such as in #13363 (closed)) are gone and it should "just work". This means that potentially more and more accurate warnings are generated, which might lead to compilation errors with Werror
.
An example from the wild is this catchall match which is now correctly flagged as redundant, because Con'
, App
and Fun
form a COMPLETE set.
Deletion of redundant matches should always get rid of the warning without introducing new inexhaustiveness warnings (or, worse, introducing inexhaustiveness crashes at runtime without producing a warning).
Apart from that, redundancy checking around pattern guards improved, all while the performance of the checker increased.
Library changes
ghc8.10
New hierarchical module structure
TODO: Mention https://gitlab.haskell.org/ghc/ghc/wikis/moduledependencies/hierarchical
templatehaskell2.16.0.0
Lift
changes
The Lift
class features a new method liftTyped :: t > Q (TExp t)
. To facilitate making it easier to define implementations of lift
in terms of liftTyped
, the default implementation of lift
has been changed. Before, the default implementation used the Data
type class:
default lift :: Data t => t > Q Exp
lift = liftData
The new default implementation is in terms of liftTyped
:
default lift :: (r ~ 'LiftedRep) => t > Q Exp
lift = unTypeQ . liftTyped
This will affect any Lift
instance that does not explicitly define lift
. There are two ways to adapt to this change:
 Derive the
Lift
instance using theDeriveLift
extension, which has been available since GHC 8.0.  Explicitly define
liftTyped
, guarding it with#if MIN_VERSION_template_haskell(2,16,0) ... #endif
if you want to preserve backwards compatibility with older versions oftemplatehaskell
.
TupE
/UnboxedTupE
changes
The types of the TupE
and UnboxedTupE
constructors have changed:
data Exp
= ...
  TupE [Exp]
  UnboxedTupE [Exp]
+  TupE [Maybe Exp]
+  UnboxedTypE [Maybe Exp]
This is because TupE
and UnboxedTupE
are now capable of handling tuple sections. For example, (1,)
is represented with TupE [Just (LitE (IntegerL 1)), Nothing]
as an Exp
.
Unary tuple changes
Using TupleT 1
, TupE [exp]
, or TupP [pat]
will now produce unary tuples (i.e., involving the Unit
type from GHC.Tuple
) instead of silently dropping the parentheses. This brings Template Haskell's treatment of boxed tuples in line with that of unboxed tuples, as UnboxedTupleT
, UnboxedTupE
, and UnboxedTupP
also produce unary unboxed tuples (i.e., Unit#
) when applied to only one argument.