3. Release notes for version 8.8.1 — Glasgow Haskell Compiler 8.8.4 User's Guide

# Glasgow Haskell Compiler User's Guide

# 3. Release notes for version 8.8.1

The significant changes to the various parts of the compiler are listed in the following sections. There have also been numerous bug fixes and performance improvements over the 8.6.1 release.

## 3.1. Highlights

The highlights, since the 8.6.1 release, are:

- GHC now supports visible type applications at the type level.

- Type variables in type family instances and rewrite rules can now be explicitly `forall`-bound.

- A new code layout algorithm for x86.

- The final phase of the `MonadFail` proposal has been implemented.

- Many, many bug fixes.

## 3.2. Full details

### 3.2.1. Language

- GHC now supports visible kind applications, as described in GHC proposal #15. This extends the existing visible type applications feature to permit type applications at the type level (e.g., `f :: Proxy ('Just @Bool 'True)`) in addition to the term level (e.g., `g = Just @Bool True`).

- GHC now allows explicitly binding type variables in type family instances and rewrite rules, as described in GHC proposal #7. For instance:

      type family G a b where
        forall x y. G [x] (Proxy y) = Double
        forall z.   G z   z         = Bool
      {-# RULES "example" forall a. forall (x :: a). id x = x #-}

- `ScopedTypeVariables`: The type variable that a type signature on a pattern can bring into scope can now stand for arbitrary types. Previously, they could only stand in for other type variables, but this restriction was deemed unnecessary in GHC proposal #29. Also see Issue #15050.

- The pattern-match coverage checker now checks for cases that are unreachable due to constructors have strict argument types. For instance, in the following example:

      data K = K1 | K2 !Void

      f :: K -> ()
      f K1 = ()

  `K2` cannot be matched on in `f`, since it is impossible to construct a terminating value of type `Void`. Accordingly, GHC will not warn about `K2` (whereas previous versions of GHC would).

- `(!)` and `(.)` are now valid type operators:

      type family a ! b
      type family a . b

- `forall` is now always a keyword in types to provide more helpful error messages when `-XExplicitForall` is off.

- An existential context no longer requires parenthesization:

      class a + b
      data D1 = forall a b. (a + b) => D1 a b
      data D2 = forall a b.  a + b  => D2 a b -- now allowed

- `{-# UNPACK #-}` annotation no longer requires parenthesization:

      data T = MkT1 { a :: {-# UNPACK #-} (Maybe Int && Bool) }
             | MkT2 { a :: {-# UNPACK #-}  Maybe Int && Bool  } -- now allowed

      data G where
        MkG1 :: {-# UNPACK #-} (Maybe Int && Bool) -> G
        MkG2 :: {-# UNPACK #-}  Maybe Int && Bool  -> G  -- now allowed

- The requirement that kind signatures always be parenthesized has been relaxed. For instance, it is now permissible to write `Proxy '(a :: A, b :: B)` (previous GHC versions required extra parens: `Proxy '((a :: A), (b :: B))`).

- `-Woverflowed-literals` checks all literals. Previously, it would only inspect boxed expression literals.

- `-Wempty-enumerations` now also works for `Numeric.Natural`.

### 3.2.2. Compiler

- The final phase of the `MonadFail` proposal has been implemented. Accordingly, the `MonadFailDesugaring` language extension is now deprecated, as its effects are always enabled. Similarly, the `-Wnoncanonical-monadfail-instances` flag is also deprecated, as there is no longer any way to define a “non-canonical” `Monad` or `MonadFail` instance.

- New `-keep-hscpp-files` to keep the output of the CPP pre-processor.

- The `-Wcompat` warning group now includes `-Wstar-is-type`.

- The `-fllvm-pass-vectors-in-regs` flag is now deprecated as vector arguments are now passed in registers by default.

- The `-fblock-layout-cfg` flag enables a new code layout algorithm on x86. This is enabled by default at `-O` and `-O2`.

- The deprecated ghc-flag `-Wamp` has been removed.

- Add new `-Wmissing-deriving-strategies` flag that warns users when they are not taking advantage of `DerivingStrategies`. The warning is supplied at each `deriving` site.

- When loading modules that use `UnboxedTuples` into GHCi, it will now automatically enable -fobject-code for these modules and all modules they depend on. Before this change, attempting to load these modules into the interpreter would just fail, and the only convenient workaround was to enable -fobject-code for all modules.

- Add support for generating a new type of output: extended interfaces files. Generation of these files, which sport a `.hie` suffix, is enabled via the `-fwrite-ide-info` flag. See Options related to extended interface files for more information.

### 3.2.3. LLVM backend

The `LLVM backend` of this release is compatible with LLVM 7.

### 3.2.4. Runtime system

- Add and document new FFI functions `hs_lock_stable_ptr_table` and `hs_unlock_stable_ptr_table`. These replace the undocumented functions `hs_lock_stable_tables` and `hs_unlock_stable_tables`, respectively. The latter should now be considered deprecated.

- Document the heretofore undocumented FFI function `hs_free_stable_ptr_unsafe`, used in conjunction with manual locking and unlocking.

- The runtime linker on Windows has been overhauled to properly handle section alignment, lower the amount of wasted memory and lower the amount of in use memory. See Issue #13617. Note that committed memory may be slightly higher.

- The output filename used for eventlog output can now be specified with the `-ol` flag.

### 3.2.5. Template Haskell

- Reifying type classes no longer shows redundant class type variables and contexts in the type signature of each class method. For instance, reifying the following class:

      class C a where
        method :: a

  Used to produce the following:

      class C a where
        method :: forall a. C a => a

  Where the `forall a. C a =>` part is entirely redundant. This part is no longer included when reifying `C`. It’s possible that this may break some code which assumes the existence of `forall a. C a =>`.

- Template Haskell has been updated to support visible kind applications and explicit `foralls` in type family instances and `RULES`. These required a couple of backwards-incompatible changes to the `template-haskell` API. Please refer to the GHC 8.8 Migration Guide for more details.

- Template Haskell now supports implicit parameters and recursive do.

- Template Haskell splices can now embed assembler source (Issue #16180)

### 3.2.6. `ghc-prim` library

- GHC now exposes a new primop, `traceBinaryEvent#`. This primop writes eventlog events similar to `traceEvent#` but allows the user to pass the event payload as a binary blob instead of a zero-terminated `ByteString`.

- The `StableName#` type parameter now has a phantom role instead of a representational one. There is really no reason to care about the type of the underlying object.

### 3.2.7. `ghc` library

- Continued refinemnt of Trees That Grow AST representation.

### 3.2.8. `base` library

- The final phase of the `MonadFail` proposal has been implemented. As a result of this change:

  - The `fail` method of `Monad` has been removed in favor of the method of the same name in the `MonadFail` class.

  - `MonadFail(fail)` is now re-exported from the `Prelude` and `Control.Monad` modules.

  These are breaking changes that may require you to update your code. Please refer to the GHC 8.8 Migration Guide for more details.

- Support the characters from recent versions of Unicode (up to v. 12) in literals

    (see Issue #5518).

- The `StableName` type parameter now has a phantom role instead of a representational one. There is really no reason to care about the type of the underlying object.

- The functions `zipWith3` and `zip3` in `Prelude` can now fuse, together with `zipWith4` to `zipWith7` as well as their tuple counterparts in `Data.List`.

### 3.2.9. Build system

- Configure: Add ALEX and HAPPY variables to explicitly set the alex and happy programs to use.

- Configure: Deprecate –with-ghc=ARG in favour of the GHC variable.

## 3.3. 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.

Package | Version | Reason for inclusion
--- | --- | ---
ghc | 8.8.4 | The compiler itself
Cabal | 3.0.1.0 | Dependency of `ghc-pkg` utility
Win32 | 2.6.1.0 | Dependency of `ghc` library
array | 0.5.4.0 | Dependency of `ghc` library
base | 4.13.0.0 | Core library
binary | 0.8.7.0 | Dependency of `ghc` library
bytestring | 0.10.10.1 | Dependency of `ghc` library
containers | 0.6.2.1 | Dependency of `ghc` library
deepseq | 1.4.4.0 | Dependency of `ghc` library
directory | 1.3.6.0 | Dependency of `ghc` library
filepath | 1.4.2.1 | Dependency of `ghc` library
ghc-boot-th | 8.8.4 | Internal compiler library
ghc-boot | 8.8.4 | Internal compiler library
ghc-compact | 0.1.0.0 | Core library
ghc-heap | 8.8.4 | GHC heap-walking library
ghc-prim | 0.5.3 | Core library
ghci | 8.8.4 | The REPL interface
haskeline | 0.7.5.0 | Dependency of `ghci` executable
hpc | 0.6.0.3 | Dependency of `hpc` executable
integer-gmp | 1.0.2.0 | Core library
libiserv | 8.8.4 | Internal compiler library
mtl | 2.2.2 | Dependency of `Cabal` library
parsec | 3.1.14.0 | Dependency of `Cabal` library
pretty | 1.1.3.6 | Dependency of `ghc` library
process | 1.6.9.0 | Dependency of `ghc` library
stm | 2.5.0.0 | Dependency of `haskeline` library
template-haskell | 2.15.0.0 | Core library
terminfo | 0.4.1.4 | Dependency of `haskeline` library
text | 1.2.4.0 | Dependency of `Cabal` library
time | 1.9.3 | Dependency of `ghc` library
transformers | 0.5.6.2 | Dependency of `ghc` library
unix | 2.7.2.2 | Dependency of `ghc` library
xhtml | 3000.2.2.1 | Dependency of `haddock` executable