3. Release notes for version 8.4.1 — Glasgow Haskell Compiler 8.4.0.20180224 User's Guide

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# 3. Release notes for version 8.4.1

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 8.2.1 release.

Note

This compiling this release requires GCC 4.7 or newer due to Trac #14244.

## 3.1. Highlights

The highlights, since the 8.2.1 release, are:

- GHC is now capable of deriving more instances
- More refinement of the `-XTypeInType` story and improvements in type error messages.
- Further improvements in code generation
- Many, many bug fixes.

## 3.2. Full details

### 3.2.1. Language

- Data families have been generalised a bit: a data family declaration can now end with a kind variable `k` instead of `Type`. Additionally, data/newtype instance no longer need to list all the patterns of the family if they don’t wish to; this is quite like how regular datatypes with a kind signature can omit some type variables.

- There are now fewer restrictions regarding whether kind variables can appear on the right-hand sides of type and data family instances. Before, there was a strict requirements that all kind variables on the RHS had to be explicitly bound by type patterns on the LHS. Now, kind variables can be implicitly bound, which allows constructions like these:

      data family Nat :: k -> k -> *
      -- k is implicitly bound by an invisible kind pattern
      newtype instance Nat :: (k -> *) -> (k -> *) -> * where
        Nat :: (forall xx. f xx -> g xx) -> Nat f g

      class Funct f where
        type Codomain f :: *
      instance Funct ('KProxy :: KProxy o) where
        -- o is implicitly bound by the kind signature
        -- of the LHS type pattern ('KProxy)
        type Codomain 'KProxy = NatTr (Proxy :: o -> *)

- Implicitly bidirectional pattern synonyms no longer allow bang patterns (`!`) or irrefutable patterns (`~`) on the right-hand side. Previously, this was allowed, although the bang patterns and irrefutable patterns would be silently ignored when used in an expression context. This is now a proper error, and explicitly bidirectional pattern synonyms should be used in their stead. That is, instead of using this (which is an error):

      data StrictJust a = Just !a

  Use this:

      data StrictJust a <- Just !a where
        StrictJust !a = Just a

- GADTs with kind-polymorphic type arguments now require `-XTypeInType`. For instance, consider the following,

      data G :: k -> * where
        GInt   :: G Int
        GMaybe :: G Maybe

  In previous releases this would compile with `-XPolyKinds` alone due to bug Trac #13391. As of GHC 8.4, however, this requires `-XTypeInType`. Note that since GADT kind signatures aren’t generalized, this will also require that you provide a CUSK by explicitly quantifying over the kind argument, `k`,

      data G :: forall k. k -> * where
        GInt   :: G Int
        GMaybe :: G Maybe

- The order in which type variables are quantified in GADT constructor type signatures has changed. Before, if you had `MkT` as below:

      data T a where
        MkT :: forall b a. b -> T a

  Then the type of `MkT` would (counterintuitively) be `forall a b. b -> T a`! Now, GHC quantifies the type variables in the order that the users writes them, so the type of `MkT` is now `forall b a. b -> T a` (this matters for `-XTypeApplications`).

- The new `-XEmptyDataDeriving` extension allows deriving `Eq`, `Ord`, `Read`, and `Show` instances directly for empty data types, as in `data Empty deriving Eq`. (Previously, this would require the use of `-XStandaloneDeriving` to accomplish.)

  One can also now derive `Data` instances directly for empty data types (as in `data Empty deriving Data`) without needing to use `-XStandaloneDeriving`. However, since already requires a GHC extension (`-XDeriveDataTypeable`), one does not need to enable `-XEmptyDataDeriving` to do so. This also goes for other classes which require extensions to derive, such as `-XDeriveFunctor`.

- Hexadecimal floating point literals (e.g. `0x0.1p4`), enabled with `-XHexFloatLiterals`. See Hexadecimal floating point literals for the full details.

### 3.2.2. Compiler

- LLVM code generator (e.g. `-fllvm`) compatible with LLVM releases in the 5.0 series.

- Add warning flag `-Wmissing-export-lists` which causes the type checker to warn when a module does not include an explicit export list.

- The `configure` script now no longer accepts `--with-TOOL` flags (e.g. `--with-nm`, `--with-ld`, etc.). Instead, these are taken from environment variables, as is typical in `autoconf` scripts. For instance, `./configure --with-nm=/usr/local/bin/nm` turns into `./configure NM=/usr/local/bin/nm`.

- Derived `Functor`, `Foldable`, and `Traversable` instances are now optimized when their last type parameters have phantom roles. Specifically,

      fmap _ = coerce
      traverse _ x = pure (coerce x)
      foldMap _ _ = mempty

  These definitions of `foldMap` and `traverse` are lazier than the ones we would otherwise derive, as they may produce results without inspecting their arguments at all.

  See also Deriving Functor instances, Deriving Foldable instances, and Deriving Traversable instances.

- Derived instances for empty data types are now substantially different than before. Here is an overview of what has changed. These examples will use a running example of `data Empty a` to describe what happens when an instance is derived for `Empty`:

  - Derived `Eq` and `Ord` instances would previously emit code that used `error`:

        instance Eq (Empty a) where
          (==) = error "Void =="

        instance Ord (Empty a) where
          compare = error "Void compare"

    Now, they emit code that uses maximally defined, lazier semantics:

        instance Eq (Empty a) where
          _ == _ = True

        instance Ord (Empty a) where
          compare _ _ = EQ

  - Derived `Read` instances would previous emit code that used `parens`:

        instance Read (Empty a) where
          readPrec = parens pfail

    But `parens` forces parts of the parsed string that it doesn’t need to. Now, the derived instance will not use `parens` (that it, parsing `Empty` will always fail, without reading any input):

        instance Read (Empty a) where
          readPrec = pfail

  - Derived `Show` instances would previously emit code that used `error`:

        instance Show (Empty a) where
          showsPrec = "Void showsPrec"

    Now, they emit code that inspects the argument. That is, if the argument diverges, then showing it will also diverge:

        instance Show (Empty a) where
          showsPrec _ x = case x of {}

  - Derived `Functor`, `Foldable`, `Traversable`, `Generic`, `Generic1`, `Lift`, and `Data` instances previously emitted code that used `error`:

        instance Functor Empty where
          fmap = error "Void fmap"

        instance Foldable Empty where
          foldMap = error "Void foldMap"

        instance Traversable Empty where
          traverse = error "Void traverse"

        instance Generic (Empty a) where
          from = M1 (error "No generic representation for empty datatype Empty")
          to (M1 _) = error "No values for empty datatype Empty"
        -- Similarly for Generic1

        instance Lift (Empty a) where
          lift _ = error "Can't lift value of empty datatype Empty"

        instance Data a => Data (Empty a) where
          gfoldl _ _ _ = error "Void gfoldl"
          toConstr _ = error "Void toConstr"
          ...

    Now, derived `Functor`, `Traversable, ``Generic`, `Generic1`, `Lift`, and `Data` instances emit code which inspects their arguments:

        instance Functor Empty where
          fmap _ x = case x of {}

        instance Traversable Empty where
          traverse _ x = pure (case x of {})

        instance Generic (Empty a) where
          from x = M1 (case x of {})
          to (M1 x) = case x of {}

        -- Similarly for Generic1

        instance Lift (Empty a) where
          lift x = pure (case x of {})

        instance Data a => Data (Empty a) where
          gfoldl _ x = case x of {}
          toConstr x = case x of {}
          ...

    Derived `Foldable` instances now are maximally lazy:

        instance Foldable Empty where
          foldMap _ _ = mempty

- Derived `Foldable` instances now derive custom definitions for `null` instead of using the default one. This leads to asymptotically better performance for recursive types not shaped like cons-lists, and allows `null` to terminate for more (but not all) infinitely large structures.

- Configure on Windows now supports the `--enable-distro-toolchain` `configure` flag, which can be used to build a GHC using compilers on your `PATH` instead of using the bundled bindist. See Trac #13792

- GHC now enables `-fllvm-pass-vectors-in-regs` by default. This means that GHC will now use native vector registers to pass vector arguments across function calls.

- The optional `instance` keyword is now usable in type family instance declarations. See Trac #13747

- Lots of other bugs. See Trac for a complete list.

- New flags `-fignore-optim-changes` and `-fignore-hpc-changes` allow GHC to reuse previously compiled modules even if they were compiled with different optimisation or HPC flags. These options are enabled by default by `--interactive`. See Trac #13604

### 3.2.3. Runtime system

- Function `hs_add_root()` was removed. It was a no-op since GHC-7.2.1 where module initialisation stopped requiring a call to `hs_add_root()`.
- Proper import library support added to GHC which can handle all of the libraries produced by `dlltool`. The limitation of them needing to be named with the suffix `.dll.a` is also removed. See Trac #13606, Trac #12499, Trac #12498
- The GHCi runtime linker on Windows now supports the `big-obj` file format.
- The runtime system’s native stack backtrace support on POSIX platforms is now triggered by `SIGQUIT` instead of `SIGUSR2` as it was in previous releases. This change is to bring GHC’s behavior into compliance with the model set by the most Java virtual machine implementations.
- The GHC runtime on Windows now uses Continue handlers instead of Vectorized handlers to trap exceptions. This change gives other exception handlers a chance to handle the exception before the runtime does. Furthermore The RTS flag `--install-seh-handlers=` Can be used on Wndows to completely disable the runtime’s handling of exceptions. See Trac #13911, Trac #12110.
- The GHC runtime on Windows can now generate crash dumps on unhandled exceptions using the RTS flag `--generate-crash-dumps`.
- The GHCi runtime linker now avoid calling GCC to find libraries as much as possible by caching the list of search directories of GCC and querying the file system directly. This results in much better performance, especially on Windows.
- The GHC runtime on Windows can now generate stack traces on unhandled exceptions. When running in GHCi more information is displayed about the symbols if available. This behavior can be controlled with the RTS flag –generate-stack-traces=<yes|no>.

### 3.2.4. Template Haskell

- Template Haskell now reifies data types with GADT syntax accurately. Previously, TH used heuristics to determine whether a data type should be reified using GADT syntax, which could lead to incorrect results, such as `data T1 a = (a ~ Int) => MkT1` being reified as a GADT and `data T2 a where MkT2 :: Show a => T2 a` not being reified as a GADT.

  In addition, reified GADT constructors now more accurately track the order in which users write type variables. Before, if you reified `MkT` as below:

      data T a where
        MkT :: forall b a. b -> T a

  Then the reified type signature of `MkT` would have been headed by `ForallC [PlainTV a, PlainTV b]`. Now, reifying `MkT` will give a type headed by `ForallC [PlainTV b, PlainTV a]`, as one would expect.

- `Language.Haskell.TH.FamFlavour`, which was deprecated in GHC 8.2, has been removed.

### 3.2.5. `ghc` library

- hsSyn Abstract Syntax Tree (AST) is now extensible via the mechanism described in Trees that Grow

  The main change for users of the GHC API is that the AST is no longer indexed by the type used as the identifier, but by a specific index type,

      type GhcPs   = GhcPass 'Parsed      -- Old 'RdrName' type param
      type GhcRn   = GhcPass 'Renamed     -- Old 'Name' type param
      type GhcTc   = GhcPass 'Typechecked -- Old 'Id' type para,
      type GhcTcId = GhcTc                -- Old 'TcId' type param

  The simplest way to support the current GHC as well as earlier ones is to define

      #if MIN_VERSION_ghc(8,3,0)
      type ParseI     = GhcPs
      type RenameI    = GhcRn
      type TypecheckI = GhcTc
      #else
      type ParseI     = RdrName
      type RenameI    = Name
      type TypecheckI = Var
      #endif

  and then replace all hardcoded index types accordingly. For polymorphic types, the constraint

      #if MIN_VERSION_ghc(8,3,0)
      -- |bundle up the constraints required for a trees that grow pass
      type IsPass pass = (DataId pass, OutputableBndrId pass, SourceTextX pass)
      else
      type IsPass pass = (DataId pass, OutputableBndrId pass)
      #endif

  can be used.

### 3.2.6. `base` library

- Blank strings can now be used as values for environment variables using the `System.Environment.Blank` module. See Trac #12494
- `Data.Type.Equality.==` is now a closed type family. It works for all kinds out of the box. Any modules that previously declared instances of this family will need to remove them. Whereas the previous definition was somewhat ad hoc, the behavior is now completely uniform. As a result, some applications that used to reduce no longer do, and conversely. Most notably, `(==)` no longer treats the `*`, `j -> k`, or `()` kinds specially; equality is tested structurally in all cases.

### 3.2.7. Build system

- `dll-split` has been removed and replaced with an automatic partitioning utility `gen-dll`. This utility can transparently split and compile any DLLs that require this. Note that the `rts` and `base` can not be split at this point because of the mutual recursion between `base` and `rts`. There is currently no explicit dependency between the two in the build system and such there is no way to notify `base` that the `rts` has been split, or vice versa. (see Trac #5987).

## 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 | 8.4.0.20180224 | The compiler itself
Cabal | 2.1.0.0 | Dependency of `ghc-pkg` utility
Win32 | 2.6.1.0 | Dependency of `ghc` library
array | 0.5.2.0 | Dependency of `ghc` library
base | 2.1 | Core library
binary | 0.8.5.1 | Dependency of `ghc` library
bytestring | 0.10.8.2 | Deppendency of `ghc` library
containers | 0.5.11.0 | Dependency of `ghc` library
deepseq | 1.4.3.0 | Dependency of `ghc` library
directory | 1.3.1.5 | Dependency of `ghc` library
filepath | 1.4.2 | Dependency of `ghc` library
ghc-boot | 8.4.0.20180224 | Internal compiler library
ghc-compact | 0.1.0.0 | Core library
ghc-prim | 2.1 | Core library
ghci | 8.4.0.20180224 | The REPL interface
haskeline | 0.7.4.2 | Dependency of `ghci` executable
hpc | 0.6.0.3 | Dependency of `hpc` executable
integer-gmp | 1.0.1.0 | Core library
mtl | 2.2.2 | Dependency of `Cabal` library
parsec | 1.12 | Dependency of `Cabal` library
process | 1.6.3.0 | Dependency of `ghc` library
template-haskell | 2.13.0.0 | Core library
text | 1.2.3.0 | Dependency of `Cabal` library
time | 1.8.0.2 | Dependency of `ghc` library
transformers | 0.5.5.0 | Dependency of `ghc` library
unix | 2.7.2.2 | Dependency of `ghc` library
xhtml | 3000.2.2 | Dependency of `haddock` executable

### 3.3.1. Win32

Attention

This release is a backwards incompatible release which corrects the type of certain APIs. See issue #24.

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