3. Release notes for version 8.6.1 — Glasgow Haskell Compiler 8.6.4 User's Guide

# Glasgow Haskell Compiler User's Guide

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# 3. Release notes for version 8.6.1

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 8.4.1 release.

## 3.1. Highlights

The highlights, since the 8.4.1 release, are:

- Support for `QuantifiedConstraints`.
- A new flexible deriving scheme, `DerivingVia`.
- A new plugin mechanism and support for plugins to modify their effect on GHC’s recompilation checking logic.
- Valid hole fits in error messages.
- A number of syntactic language extensions.
- Programs are no longer constrained by the Windows `MAX_PATH` file path length limit. The file path limit is now approximately 32,767 characters. Note that GHC itself is still somewhat limited due to GCC not supporting file namespaced paths. Paths that are passed directly to the compiler, linker or other GNU tools are currently still constrained. See File paths under Windows for details.
- Many, many bug fixes.

## 3.2. Full details

### 3.2.1. Language

- Use of quantified type variables in constraints is now allowed via the `QuantifiedConstraints` language extension. This long-awaited feature enables users to encode significantly more precision in their types. For instance, the common `MonadTrans` typeclass could now make the expectation that an applied transformer is must be a `Monad`

      class (forall m. Monad m => Monad (t m)) => MonadTrans t where {- ... -}

  Additionally, quantification can enable terminating instance resolution where this previously was not possible. See Quantified constraints for details.

- A new `DerivingVia` language extension has been added which allows the use of the `via` deriving strategy. For instance:

      newtype T = MkT Int
        deriving Monoid via (Sum Int)

  See Deriving via for more information.

- A new `StarIsType` language extension has been added which controls whether `*` is parsed as `Data.Kind.Type` or a regular type operator. `StarIsType` is enabled by default.

- GHC now permits the use of a wildcard type as the context of a standalone `deriving` declaration with the use of the `PartialTypeSignatures` language extension. For instance, this declaration:

      deriving instance _ => Eq (Foo a)

  Denotes a derived `Eq (Foo a)` instance, where the context is inferred in much the same way as ordinary `deriving` clauses do. See Partial Type Signatures.

- Data declarations with empty `where` clauses are no longer valid without the extension `GADTSyntax` enabled. For instance, consider the following,

      data T where

  The grammar is invalid in Haskell2010. Previously it could be compiled successfully without `GADTs`. As of GHC 8.6.1, this is a parse error.

- Incomplete patterns warning `-Wincomplete-patterns` is extended to guards in pattern bindings and `if` alternatives of `MultiWayIf`. For instance, consider the following,

      foo :: Bool -> Int
      foo b = if | b -> 1

  In GHC 8.6.1, it will raise the warning:

      <interactive>:2:12: warning: [-Wincomplete-patterns]
          Pattern match(es) are non-exhaustive
          In a multi-way if alternative:
              Guards do not cover entire pattern space

  See Trac #14773.

- Scoped type variables now work in default methods of class declarations and in pattern synonyms in Template Haskell. See Trac #14885.

- `do` expressions, lambda expressions, etc. to be directly used as a function argument, enabled with `BlockArguments`. See More liberal syntax for function arguments for the full details.

- Underscores in numeric literals (e.g. `1_000_000`), enabled with `NumericUnderscores`. See Numeric underscores for the full details.

- CUSKs now require all kind variables to be explicitly quantified. This was already the case with `TypeInType`, but now `PolyKinds` also exhibits this behavior. This means that the following example is no longer considered to have a CUSK:

      data T1 :: k -> Type       -- No CUSK: `k` is not explicitly quantified

- Functionality of `TypeInType` has been subsumed by `PolyKinds`, and it is now merely a shorthand for `PolyKinds`, `DataKinds`, and `NoStarIsType`. The users are advised to avoid `TypeInType` due to its misleading name: the `Type :: Type` axiom holds regardless of whether it is enabled.

- GHC has become more diligent about catching illegal uses of kind polymorphism. For instance, GHC 8.4 would accept the following without the use of `PolyKinds`:

      f :: forall k (a :: k). Proxy a
      f = Proxy

  This is now an error unless `PolyKinds` is enabled.

- Type literals now could be used in type class instances without the extension `FlexibleInstances`.

  See Trac #13833.

- `MonadFailDesugaring` is now enabled by default. See MonadFail Proposal (MFP) for more details.

### 3.2.2. Compiler

- GHC now no longer adds the current file’s directory as a general include path calling the C compiler. Instead we use `-iquote` to only add it as an include path for #include “”. See Trac #14312.
- GHC now supports British spelling of `GeneralizedNewtypeDeriving`.
- GHC now does significantly more constant folding in its core-to-core optimiser. This will result in significantly better code being generated for some programs. See Trac #9136.
- GHC now offers significantly more information about typed holes such as valid hole fits and refinement hole fits. See Valid Hole Fits for more information.
- The code-generation effects of `-dynamic` can now be enabled independently by the flag `-fexternal-dynamic-refs`. If you don’t know why you might need this, you don’t need it.
- `-Wcompat` now includes `-Wimplicit-kind-vars` to provide early detection of breakage that will be caused by implementation of GHC proposal #24 in a future release.
- The `-package-env ⟨file⟩|⟨name⟩` flag and `GHC_ENVIRONMENT` environment variable now accept the `-` value, which instructs GHC to ignore any package environment files.

### 3.2.3. Plugins

- GHC’s plugin mechanism now offers plugin authors control over their plugin’s effect on recompilation checking. Specifically the `Plugin` record name has a new field

      data Plugin = Plugin {
          pluginRecompile :: [CommandLineOption] -> IO PluginRecompile
        , {- ... -}
        }

      data PluginRecompile = ForceRecompile | NoForceRecompile | MaybeRecompile Fingerprint

  Plugin based on `defaultPlugin` will have their previous recompilation behavior (`ForceRecompile`) preserved. However, plugins that are “pure” are encouraged to override this to either `NoForceRecompile` or `MaybeRecompile`. See Controlling Recompilation for details.

- GHC now provides a class of new plugins: source plugins. These plugins can inspect and modify a variety of intermediate representations used by the compiler’s frontend. These include:

  > - The ability to modify the parser output
  > - The ability to inspect the renamer output
  > - The ability to modify the typechecked AST
  > - The ability to modify Template Haskell splices
  > - The ability to modify interface files as they are loaded

  See Source plugins for details.

### 3.2.4. GHCi

- Added an experimental `:doc` command that displays the documentation for a declaration.

### 3.2.5. Runtime system

- The GHC runtime linker now prefers user shared libraries above system ones. When extra search directories are specified these are searched before anything else. This fixes `iuuc` on Windows given the proper search directories (e.g `-L/mingw64/lib`).
- The GHC runtime linker now uses `LIBRARY_PATH` and the runtime loader now also searches `LD_LIBRARY_PATH`.
- The GHC runtime on Windows is no longer constrained by the `MAX_PATH` file path length limitation. See File paths under Windows.
- The runtime now allows use of the `-hT` profiling variety on programs built with `-prof`.
- The STM assertions mechanism (namely the `always` and `alwaysSucceeds` functions) has been removed. This happened a bit earlier than proposed in the deprecation pragma included in GHC 8.4, but due to community feedback we decided to move ahead with the early removal.

### 3.2.6. Template Haskell

### 3.2.7. `ghc` library

- The `Plugin` record now has a several new fields for the new source plugins and recompilation checking mechanisms.

### 3.2.8. `base` library

- `($!)` is now representation-polymorphic like `($)`.
- The module `Data.Functor.Contravariant` has been moved from the `contravariant` package into `base`. All the other modules in `contravariant` (`Data.Functor.Contravariant.Divisible`, etc.) have not been moved to `base`, and they still reside in `contravariant`.

### 3.2.9. `ghc-prim` library

- Added new `addWordC#` operation for unsigned addition with carry.

### 3.2.10. Build system

## 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 | 8.6.4 | The compiler itself
Cabal | 2.4.0.1 | Dependency of `ghc-pkg` utility
Win32 | 2.6.1.0 | Dependency of `ghc` library
array | 0.5.3.0 | Dependency of `ghc` library
base | 4.12.0.0 | Core library
binary | 0.8.6.0 | Dependency of `ghc` library
bytestring | 0.10.8.2 | Deppendency of `ghc` library
containers | 0.6.0.1 | Dependency of `ghc` library
deepseq | 1.4.4.0 | Dependency of `ghc` library
directory | 1.3.3.0 | Dependency of `ghc` library
filepath | 1.4.2.1 | Dependency of `ghc` library
ghc-boot | 8.6.4 | Internal compiler library
ghc-compact | 0.1.0.0 | Core library
ghc-prim | 0.5.3 | Core library
ghci | 8.6.4 | The REPL interface
haskeline | 0.7.4.3 | Dependency of `ghci` executable
hpc | 0.6.0.3 | Dependency of `hpc` executable
integer-gmp | 1.0.2.0 | Core library
mtl | 2.2.2 | Dependency of `Cabal` library
parsec | 3.1.13.0 | Dependency of `Cabal` library
process | 1.6.5.0 | Dependency of `ghc` library
template-haskell | 2.14.0.0 | Core library
text | 1.2.3.1 | Dependency of `Cabal` library
time | 1.8.0.2 | 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

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