# Release notes for version 5.04

## User-visible compiler changes

- Full support for MacOS X, including fully optimized compilation, has been added. Only a native code generator and support for `-split-objs` is still missing. Everything else needs more testing, but should work.

- `ghc-pkg`: new options `--auto-ghci-libs`, `-u`/`--update-package`, `--force`, and `-i`/`--input-file`, and suppport for expanding environment variables in package descriptions. See ).

- The latest version of the FFI spec is fully supported. The syntax of FFI declarations has changed accordingly. The old syntax is still accepted for the time being, but will elicit a warning from the compiler.

- New option: `-F` specifies a user-defined preprocessing phase (see ).

- Major overhaul of the heap profiling subsystem, with new facilities for retainer profiling and biographical profiling (ala nhc98, albeit with a couple of omissions). The syntax of the runtime heap-profiling options has changed. See .

- The type system now supports full rank-N types (previously only limited rank-2 types were supported). See .

- Explicit kind annotations can now be given on any binding occurrence of a type variable. See .

- The handling of type synonyms has been rationalised. See .

- Fixes for several space leaks in the compiler itself (these fixes were also merged into 5.02.3).

- It is now possible to derive arbitrary classes for newtypes. See .

- Deadlock is now an exception, rather than a return status from the scheduler. See the module `Control.Exception` in the library documentation for more details.

- The syntax and behaviour of `RULE` pragmas has changed slightly. See .

- Interface files are now in a binary format to reduce compilation times. To view an interface file in plain text, use the `--show-iface` flag.

- A restriction on the form of class declarations has been lifted. In Haskell 98, it is illegal for class method types to mention constraints on the class type variable. eg.

         class Seq s a where
            elem     :: Eq a => a -> s a -> Bool

  This restriction has now been lifted in GHC.

- Main threads can now receive the `BlockedOnDeadMVar` exception in the same way as other threads.

- The `-fall-strict` flag never really worked, and has been removed.

- The syntax of `.hi-boot` files is now much clearer and Haskell-like. See .

- There is a new flag `-fffi` which enables FFI support without turning on the rest of the GHC extensions.

- The syntax for implicit parameter bindings has changed. Previously the keyword `with` was used to introduce implicit bindings, but now implicit bindings may be introduced using `let` (see ). As a result of this, `with` is no longer a keyword when `-fglasgow-exts` is turned on.

  The option `-fwith` may be used to restore the old behaviour.

- Infix type constructors are now allowed, and must begin with a colon (as with data constructors). See .

- The `do`-notation syntax is now rebindable in the same way as other built-in syntax. See .

- Support for using “frameworks” on Darwin/MacOS X has been added. See the `-framework` option in , and the `framework_dirs` field of a package spec in .

## User-visible interpreter (GHCi) changes

- New commands: `:browse`, `:set args`, `:set prog`, `:show bindings`, and `:show modules` (see ).

- There is a much more flexible mechanism for manipulating the scope for expressions typed at the prompt. For example, one can now have both the `Prelude` and the exports of several compiled modules in scope at the same time. See .

- GHCi now supports `foreign import "wrapper"` FFI declarations.

## User-visible library changes

- GHC is in the process of moving to a new hierarchical set of libraries. At the moment, we have two sets of libraries, both described in accompanying documents:

  - The “new libraries” which are hierarchical and consist of the following packages: `base`, `haskell98`, `haskell-src`, and `network`. Broadly speaking, `base` contains the `Prelude`, standard libraries and most of the contents of the old `lang` package. By default, the `base` and `haskell98` packages are enabled.

  - The `hslibs`, most of which are now deprecated. Where possible, new code should be written to use the new libraries instead.

    The following libraries in `hslibs` have not moved yet:

    - The packages `win32`, `xlib`, `graphics`, and `posix`.

    - The Edison libraries in the `data` package.

    - In the `lang` package, the modules `TimeExts`, `DirectoryExts`, `SystemExts`, and `NumExts`.

    - The HaXml libraries in the `text` package.

    - In the `util` package, the modules `MD5`, `Select`, `Memo`, `Observe`, and `Readline`.

    All other libraries from `hslibs` either have equivalents in the new libraries (see the `hslibs` docs for details), or were already deprecated and hence were not moved into the new hierarchy.

- The `Read` class is now based on a parsing combinator library which is vastly more efficient than the previous one. See the modules `Text.Read`. `Text.ParserCombinators.ReadP`, and `Text.ParserCombinators.ReadPrec` in the library documentation.

  The code generated by the compiler for derived `Read` instances should be much shorter than before.

## New experimental features

- Linear implicit parameters. See .

- The RTS has support for running in a multi-threaded environment and making non-blocking (from Haskell's point of view) calls to foreign C functions which would normally block. To enable this behaviour, configure with the `--enable-threaded-rts` option.

- The compiler can now read in files containing Core syntax (such as those produced by the `-fext-core` option) and compile them. Input files with the `.hcr` file extension are assumed to contain Core syntax.

## Internal changes

- Happy 1.13 is now required to build GHC, because of the change in names of certain libraries.