2.1. Version 9.6.1

The significant changes to the various parts of the compiler are listed in the following sections. See the migration guide on the GHC Wiki for specific guidance on migrating programs to this release.

The LLVM backend of this release is to be used with LLVM 11, 12, 13, 14 or 15.

2.1.1. Language

  • GHC is now more conservative when solving constraints that arise from superclass expansion in terms of other constraints that also arise from superclass expansion.

    For example:

    class C a
    class C a => D a b
    instance D a a => D a b

    When typechecking the instance, we need to also solve the constraints arising from the superclasses of D; in this case, we need C a. We could obtain evidence for this constraint by expanding the superclasses of the context, as D a a also has a superclass context of C a. However, is it unsound to do so in general, as we might be assuming precisely the predicate we want to prove! This can lead to programs that loop at runtime.

    When such potentially-loopy situations arise, GHC now emits a warning. In future releases, this behaviour will no longer be supported, and the typechecker will outright refuse to solve these constraints, emitting a Could not deduce error.

    In practice, you should be able to fix these issues by adding the necessary constraint to the context, e.g. for the above example:

    instance (C a, D a a) => D a b
  • Record updates for GADTs and other existential datatypes are now fully supported.

    For example:

    data D b where
      MkD :: { fld1 :: a -> a, fld2 :: a -> (), fld3 :: b } -> D b
    foo :: D b -> D b
    foo d = d { fld1 = id, fld2 = const () }

    In this example, we have an existential variable a, and we update all fields whose type involves a at once, so the update is valid.

    A side-effect of this change is that GHC now rejects some record updates involving fields whose types contain type families (these record updates were previously erroneously accepted).


    type family F a where
      F Int   = Char
      F Float = Char
    data T b = MkT { x :: [Int], y :: [F b] }
    emptyT :: forall b. T b
    emptyT = MkT [] []
    bar :: T Int
    bar = emptyT { x = [3] }

    In this example, we can’t infer the type of emptyT in bar: it could be T Int, but it could also be T Float because the type family F is not injective and T Float ~ T Int. Indeed, the following typechecks

    baz :: T Int
    baz = case ( emptyT :: T Float ) of { MkT _ y -> MkT [3] y }

    This means that the type of emptyT is ambiguous in the definition of bar above, and thus GHC rejects the record update:

    Couldn't match type `F b0' with `Char'
    Expected: [F Int]
      Actual: [F b0]
    NB: ‘F’ is a non-injective type family
    The type variable ‘b0’ is ambiguous

    To fix these issues, add a type signature to the expression that the record update is applied to (emptyT in the example above), or add an injectivity annotation to the type family in the case that the type family is in fact injective.

  • Error messages are now assigned unique error codes, of the form [GHC-12345].

  • GHC Proposal #106 has been implemented, introducing a new language extension TypeData. This extension permits type data declarations as a more fine-grained alternative to DataKinds.

  • GHC now does a better job of solving constraints in the presence of multiple matching quantified constraints. For example, if we want to solve C a b Int and we have matching quantified constraints:

    forall x y z. (Ord x, Enum y, Num z) => C x y z
    forall u v. (Enum v, Eq u) => C u v Int

    Then GHC will use the second quantified constraint to solve C a b Int, as it has a strictly weaker precondition.

  • GHC proposal #170 Unrestricted OverloadedLabels has been implemented. This extends the variety syntax for constructing labels under OverloadedLabels. Examples of newly allowed syntax: - Leading capital letters: #Foo equivalant to getLabel @”Foo” - Numeric characters: #1728 equivalent to getLabel @”1728” - Arbitrary strings: #”Hello, World!” equivalent to getLabel @”Hello, World!”

2.1.2. Compiler

  • The WebAssembly backend has been merged. This allows GHC to be built as a cross-compiler that targets wasm32-wasi and compiles Haskell code to self-contained WebAssembly modules that can be executed on a variety of different runtimes. There are a few caveats to be aware of:
    • To use the WebAssembly backend, one would need to follow the instructions on ghc-wasm-meta. The WebAssembly backend is not included in the GHC release bindists for the time being, nor is it supported by ghcup or stack yet.
    • The WebAssembly backend is still under active development. It’s presented in this GHC version as a technology preview, bugs and missing features are expected.
  • The JavaScript backend has been merged. GHC is now able to be built as a cross-compiler targeting the JavaScript platform. The backend should be considered a technology preview. As such it is not ready for use in production, is not distributed in the GHC release bindists and requires the user to manually build GHC as a cross-compiler. See the JavaScript backend wiki page on the GHC wiki for the current status, project roadmap, build instructions and demos.
  • GHC now offers a new flag, -fprefer-byte-code, which instructs the compiler to to use byte-code when available when loading home package modules for execution (e.g. when evaluating TH splices). This avoids the considerable code generation and linking costs of native code, which is often unnecessary for one-off Template Haskell splices. See this blog post for details.
  • The TypeInType is now marked as deprecated. Its meaning has been included in PolyKinds and DataKinds.
  • The -Woperator-whitespace warning no longer ignores constructor symbols (operators starting with :).
  • The -Wstar-is-type warning is now enabled by default, continuing the implementation of GHC proposal #143.

2.1.3. GHCi

  • GHCi will now accept any file-header pragmas it finds, such as {-# OPTIONS_GHC ... #-} and {-# LANGUAGE ... #-} (see Pragmas). For example, instead of using :set to enable -Wmissing-signatures, you could instead write:

    ghci> {-# OPTIONS_GHC -Wmissing-signatures #-}

This can be convenient when pasting large multi-line blocks of code into GHCi.

2.1.4. Runtime system

  • The Delimited continuation primops proposal has been implemented, adding native support for first-class, delimited continuations to the RTS. For the reasons given in the proposal, no safe API to access this functionality is provided anywhere in base. Instead, the prompt# and control0# primops are intended to be consumed by library authors directly, who may wrap them a safe API that maintains the necessary invariants. See the documentation in GHC.Prim for more details.
  • The behaviour of the -M flag has been made more strict. It will now trigger a heap overflow if the total amount of memory used by the Haskell heap exceeds the limit. Previously only live blocks were taken into account. This makes it more likely to trigger promptly when the heap is highly fragmented.
  • Fixed a bug that sometimes caused live sparks to be GC’ed too early either during minor GC or major GC with workstealing disabled. See #22528.

2.1.5. base library

Below is a list of major changes in base. See the base changelog for a full list.

2.1.6. ghc library

  • Add Foreign.C.Types.ConstPtr was added to encode const-qualified pointer return types in foreign declarations when using CApiFFI extension.

2.1.7. Included libraries

The package database provided with this distribution also contains a number of packages other than GHC itself. See the changelogs provided with these packages for further change information.