purescript

Breaking Changes

The new functional dependencies feature fixes type inference in some cases involving multi-parameter type classes. However, due to a bug in the compiler, some of those expressions were previously type checking where they should not have. As a result, it is necessary to add functional dependencies to some classes in order to make previous code type-check in some cases. Known examples are:

• MonadEff and MonadAff
• MonadState, MonadReader, and the rest of the MTL-style classes in transformers

New Features

Data.Newtype Deriving

(@garyb)

It is now possible to derive the Newtype class for any data declaration which is a newtype, using the existing deriving instance syntax:

newtype Test = Test String

derive instance newtypeTest :: Newtype Test _

Note that the second type argument should be specified as a wildcard, and will be inferred.

Added type level string functions

(@FrigoEU)

The Prim module now defines the TypeString and TypeConcat type constructors, which can be used to build more descriptive error messages which can depend on types, using the Fail constraint:

instance cannotShowFunctions 
    :: Fail ("Function type " <> TypeString (a -> b) <> " cannot be shown.") 
    => Show (a -> b) where
  show _ = "unreachable"

infixl 6 type TypeConcat as <>

--dump-corefn

(@rightfold)

The compiler now supports the --dump-corefn option, which causes the functional core to be dumped in output/**/corefn.json. This should be useful for implementing new backends which interpret the functional core.

Newtype Deriving

(@paf31)

It is now possible to derive type class instances for newtypes, by reusing the instance for the underlying type:

newtype X = X String

derive newtype instance showX :: Show X

Note that it is possible to derive instances for multi-parameter type classes, but the newtype must only appear as the last type argument.

Allow anonymous accessor chains (_.a.b)

(@rvion)

Anonymous record accessor syntax has been extended to work with chains of one or more accessors:

getBaz = _.foo.bar.baz

Functional Dependencies (@paf31)

The type class solver now supports functional dependencies. A multi-parameter type class can define dependencies between its type arguments by using the -> operator:

class Stream el s | s -> el where
  cons :: el -> (Unit -> s) -> s
  uncons :: s -> { head :: el, tail :: s }

Here, the s and el type arguments are related by a single functional dependency, which ensures that there is at most one instance for any given type s. Alternatively, the type s determines the type el, i.e. there is an implicit function from types s to types el. This information can be used by the solver to infer types where it was previously not possible.

See the following examples for more information:

• Streams
• GHC-style generics
• Type-level arithmetic
• Heterogeneous Lists

Enhancements

• Return qualifier from explicit/hiding imports (@nwolverson)
• Verify entry points exist in psc-bundle (@kRITZCREEK)
• Improved error messages for record subsumption (@FrigoEU)

psc-ide

• Resolve types/kinds for operators (@kRITZCREEK)
• Unify Completion Commands (@kRITZCREEK)
• Parse type annotations from source files (@kRITZCREEK)
• Update pursuit JSON parsing (@nwolverson)
• Remove a pursuit workaround (@kRITZCREEK)
• Add a suggestion to the UnusedDctorImport warning (@FrigoEU)
• Return JSON errors for cycles in module dependencies (@kRITZCREEK)

Bug Fixes

• Fix usage detection for operators (@garyb)
• Fix handling of duplicate module imports in JS codegen (@garyb)
• Fix a small bug in the type pretty-printer (@paf31)
• Fix function application judgment (@paf31)
• Fix inlining for $ and # operators (@garyb)
• Fix everywhereOnTypesTopDown (@ianbollinger)
• Fix unification of string literals (@paf31)

Infrastructure

• Support aeson-1.0 (@phadej)
• Support http-client-0.5 (@phadej)
• Safer installation from source in INSTALL.md (@hdgarrood)

Implementation

• Fix most HLint warnings (@ianbollinger)
• Fixing imports (@charleso)
• Export desugarDecl from Sugar.ObjectWildcards (@rvion)
• Remove legacy ObjectGetter and update doc (@rvion)

Enhancements

• Better context information for typed hole errors (@paf31)
• Improved error messages in the constraint solver. Type class errors now include better contextual information, including smaller source spans. (@paf31)

Bug Fixes

• Decode externs with correct encoding (@natefaubion)
• Fix bad codegen for empty string fields (@LiamGoodacre, #2244)
• Instantiate types in array literals before unification (@paf31, #2252)

Other

• Upgrade to protolude 0.1.6 (@ilovezfs)
• Use latest LTS (@paf31, #2241)
• Add upper bound to http-client (@paf31, #2237)
• Combine the sdist and coverage builds. Avoid .tix files during deployment. (@paf31)

Enhancements

Goto Definition

@kRITZCREEK has added the ability to return position information for expressions in psc-ide. This can be used to implement a Goto Definition feature in IDEs which use psc-ide-server as the backend.

Evaluate PSCi expressions in the browser

(@paf31)

PSCi now features an alternative backend, which can run commands in the browser via a websocket. To use this mode, simply pass the --port option on the command line:

$ pulp psci --port 9000

and open your web browser to localhost on that port.

See https://github.com/paf31/psci-experiment for a demonstration.

psc-ide architecture changes

@kRITZCREEK has worked on changing the architecture of psc-ide generally, to load data in multiple phases and asynchronously. This enables new features like Goto Definition above.

Other

• Allow pipes version 4.2 (@felixonmars)
• Elaborate re-exports (@garyb)

Bug Fixes

psc-ide

• Fix unicode encoding of json responses (@kRITZCREEK)
• Improved handling of reexports (@kRITZCREEK)

Other

• Update Data.Function constant for prelude 1.0 (@felixSchl)
• Include position info in ScopeShadowing warning (@garyb)

PureScript 0.9.1 is a major stable release of the compiler. It removes features which were deprecated in the 0.8.x series, and contains several useful enhancements and bug fixes.

This release will be accompanied by new releases of the core libraries and a compatible version of Pulp, which have been updated to work with this version.

Due to the relatively large number of breaking changes, library authors are advised that they will probably need to update their libraries to maintain compatibility. Users may prefer to continue using version 0.8.5 until their dependencies have been updated.

Breaking Changes

Name resolving

(@garyb)

The way names are resolved has now been updated in a way that may result in some breakages. The short version is: now only names that have been imported into a module can be referenced, and you can only reference things exactly as you imported them.

Some examples:

Qualified references like Control.Monad.Eff.Console.log will no longer resolve unless there is a corresponding import Control.Monad.Eff.Console as Control.Monad.Eff.Console. Importing a module unqualified does not allow you to reference it with qualification, so import X does not allow references to X.A unless there is also an import X as X.

Although the new scheme is stricter it should be easier to understand exactly what the effect of any given import statement is. The old resolution rules for qualified names were obscure and unexpected results could arise when locally-qualified module names overlapped with "actual" module names.

Module re-exports have also been tightened up as a result of these rules. Now if module X is only imported as Y, the re-export must list module Y also. If a module is imported without being re-qualified then the original name is used.

Partial Constraints

(@garyb, @paf31)

The compiler will now generate an error for a missing Partial constraints, where it would previously have issued a warning.

Module Restrictions

(@garyb, @paf31)

• Imports must now appear before other declarations in a module.
• A source file must now contain exactly one module.

These restrictions will allow us to improve incremental build times in future, since we will only need to parse a small prefix of each file in order to figure out what needs to be rebuilt. Right now, we need to parse every file fully.

Foreign Function Interface Changes

(@paf31)

Foreign modules are now found by filename rather than by searching for a custom JavaScript comment. The foreign module is found by changing the extension of the corresponding PureScript module from .purs to .js.

This change was made to be more consistent with psc-ide, and also to adopt a simple convention which will port well to other backends.

Operator Aliases

(@garyb)

All operators must be defined as aliases from now on. That is, it is no longer valid to define an operator as a name in local scope (e.g. let (#) x y = x y in ...). This change makes it possible to generate better JavaScript code for operators, by desugaring them to the functions they alias.

Other

• Deprecated class import/export syntax has been removed (@LiamGoodacre). Classes are now imported using the class keyword, and exported similarly:

  import Prelude (class Show, show)
  

• Remove support for = in record binders (@paf31).

  Record binders such as

  f { x = 0 } = true
  

  are no longer supported. Record binders must now use : instead:

  f { x: 0 } = true
  

• Prim.Object has been renamed to Prim.Record (#1768, @paf31)

Enhancements

Programmable Type Errors

(@paf31)

Constraints can now contain type-level strings which can be used as custom error messages using the Fail constraint. For example, one can now document the fact that foreign types such as JSDate cannot be made instances of Generic:

instance dateIsNotGeneric
  :: Fail "JSDate is not Generic. Consider using Int with toEpochMilliseconds instead."
  => Generic JSDate where
    fromSpine   = crashWith "fromSpine: unreachable"
    toSpine     = crashWith "toSpine: unreachable"
    toSignature = crashWith "toSignature: unreachable"

Attempting to derive a Generic instance for a type containing JSDate will then result in

A custom type error occurred while solving type class constraints:

    JSDate is not Generic. Consider using Int with toEpochMilliseconds instead.

Typed Hole Improvements

(#2070, @paf31)

Typed hole error messages now include the types of any names in scope, to assist with type-driven development:

> :t \x -> maybe 0 ?f x
Error found:
in module $PSCI
at  line 1, column 8 - line 1, column 22

  Hole 'f' has the inferred type

    t0 -> Int

  in the following context:

    it :: Maybe t0 -> Int
    x :: Maybe t0


in value declaration it

where t0 is an unknown type

Editor Support

• The results of the last rebuild are now cached by psc-ide, which improves completion support for editor plugins. (@kRITZCREEK)
• A reset command was added to psc-ide (@kRITZCREEK)
• The compiler will now suggest replacements to address MissingTypeDeclaration and TypeWildCard warnings (@nwolverson)

PSCi Improvements

(@paf31)

• The design of PSCi has been changed to improve performance. PSCi now precompiles all dependencies and uses the same incremental rebuilding approach as psc-ide. This means that the :load and :foreign commands have been removed, since dependencies are fixed and pre-compiled when PSCi loads.
• PSCi now supports alternative base libraries such as Neon, by depending on purescript-psci-support for its supporting code.

Colors in Error Messages

Types and values will now be highlighted in error messages, when the terminal supports it (MacOS and Linux for now) (@soupi).

Type Names

Prime characters are now allowed in type names. (@garyb)

Bug Fixes

• Parser error messages inside type class and instance declarations were improved (#2128, @bmjames)
• Editor suggestions for imports now use (..) (@garyb)
• Source-spans to token end position (@nwolverson)
• Some pretty printing issues related to string literals in records were fixed (@LiamGoodacre)
• Some presentation bugs in PSCi's :show import were fixed (@LiamGoodacre)
• Parsec was updated to the latest version to fix an issue with literal parsing (#2115, @hdgarrood)
• Fixed a bug related to certain typed binders which would cause the compiler to crash (#2055, @paf31)
• As-patterns now bind less tightly (@paf31)
• More identifiers can now be parsed in FFI imports (@michaelficarra)
• Fixed a performance issue which manifested under certain conditions in psc-ide (#2064, @kika)
• Fixed a test which contained an unreliable comparison (#2093, @andyarvanitis)
• The precedence of type application was corrected (#2092, @paf31)
• An indentation bug in the parser was fixed (@DavidLindbom)
• License errors from psc-publish were improved (@hdgarrood)

Other

• The test suite now exercises various compiler warnings (@garyb)
• The test suite performance was improved by using incremental rebuilds (@paf31)
• The test suite now tests that passing tests contain a main function (@hdgarrood)
• The test suite now supports tests which use multiple files (@garyb)
• Portability of the core library test suite was improved (@bmjames)
• Performance of import elaboration was improved (@garyb)
• We now use Stack for our CI builds and release builds (#1974, @hdgarrood)
• We now use NoImplicitPrelude and enable some global extensions (@garyb)
• Type-safety in the source-level AST was improved (@garyb)
• Use HSpec for the compiler tests (@garyb)
• New Prelude names in 0.9 (@garyb)

This is pre-release software

This release is provided so that library developers can test the new compiler features.

New Features

• Fast recompilation for single files in psc-ide-server #1712 (@kRITZCREEK, @paf31)

  The pscid project makes use of this to watch files as you work and raise errors and warnings when they occur with near instant feedback.

• Operator aliases can now be declared for types #416 (@garyb)

  infixr 6 type Natural as ~>
  

• Underscore wildcards can now be used in case and if expressions #1558 (@garyb)

  case _ of
    Something -> ...
  

  -- underscores can optionally be used in any part of an `if` expression
  cond = if _ then _ else _
  picker = if _ then "x" else "y"
  

• Typed holes #1283 (@garyb)

  example :: forall a. Maybe a -> Unit
  example ma = ?umm
  

  Hole 'umm' has the inferred type
  
    Unit
  
  in value declaration example
  

  You can use any identifier name after the question mark and that will be used to label the hole in the raised error message.

Breaking changes

• Type annotations may need parentheses in some situations that they previously did not due to the introduction of type operators. For example, x :: a == y will be now parsed as x :: (a == y) instead of (x :: a) == y.

Enhancements

• Improved error messages for invalid FFI identifiers #2011 (@hdgarrood)
• psc-publish now allows publishing of packages with a valid SPDX license field in bower.json #1985 (@hdgarrood)
• Haddock markdown fix #2001 (@trofi)
• psc-ide now creates the output folder on startup if it is missing #2030 (@kRITZCREEK)

Bug Fixes

• Fixed an issue with incorrect suggestions when re-exporting modules #1862 (@garyb)
• Fixed an issue with invalid redundant import warnings #1823 (@garyb)
• Fixed an issue where DuplicateSelectiveImport would not fire when it should #2004 (@garyb)
• Fixed the error that occurs when an invalid newtype is created that belongs to a data binding group #1895 (@garyb)
• Fixed a case where re-exports included unintended exports #1872 (@garyb)
• Operator aliases can now be declared for qualified data constructors #2015 (@LiamGoodacre)
• A single hiding import will no longer raise an "unspecified imports" error #2017 (@garyb)
• Fixed a case where cycles in modules were being detected when they do not occur #2018 (@garyb)
• Various cases where files were not being read as UTF-8 on Windows were fixed #2027, #2031 (@garyb, @kRITZCREEK)
• Fixed some issues in pretty printing of records #2043 (@LiamGoodacre)
• psci now shows qualified imports correctly #2040 (@LiamGoodacre)
• Parser errors are now returned as JSON during IDE rebuild #2042 (@paf31)

This is an interim bug fix release before 0.9.0.

Enhancements

• Check that FFI imports match with implementations (@hdgarrood)

  This is technically a breaking change, since some existing code might fail to compile if it has missing FFI code (purescript-dom is an example), but these libraries should be fixed soon.

• Import helper commands in psc-ide (@kRITZCREEK)

Bug Fixes

• Disallow constraint generalization for recursive functions. (#1978, @paf31)
• Fix #1991, instantiate polymorphic types before unification (@paf31)
• Use UTF8 when writing to stdout and stderr (@garyb)
• Fix for rendered constrained types needing parens. (@LiamGoodacre)
• everythingWithScope improperly traversing binary ops (@LiamGoodacre)

Other

• Update to use language-javascript 0.6.x (@nwolverson)

Breaking Changes

• We have dropped support for GHC 7.8 and older (@hdgarrood)

Enhancements

• Infer types with class constraints (@paf31)

  For example, this simple code would previously have failed with a confusing NoInstanceFound error:

  add x y = x + y
  

  The compiler will now infer the most general type, namely forall a. (Semiring a) => a -> a -> a.

  Note that constraints can only be inferred if they only mention type variables; inference of arbitrary types in constraints is not (yet) supported. So, for example, you would still have to write a type signature for a function which had a constraint such as (MonadEff (console :: CONSOLE | eff) m).

• Default require path to ../ (@nwolverson)

  The previous default behavior was no require path prefix, which was confusing for some workflows. The new default is ../, which is the prefix used in purs-loader. This option will be removed completely in 0.9.

• Expose hiding import suggestion in JSON (@nwolverson)

• Error on missing LICENSE file or missing license field in bower.json (@faineance)

Bug Fixes

• Fix #1916 (@bagl)
• Fix detection of single open import (@garyb)
• Fix true not being treated as an infallible guard (@garyb)
• Fix pretty printer spinning (@garyb)
• Fix Windows build script (@garyb)
• Fix #1889, improve performance by avoiding whitespace operations on large strings (@paf31)

psc-ide

• Fix a crash related to error messages in the case splitting command (@kRITZCREEK)
• Escape regex characters when using the flex matcher (@kRITZCREEK)
• Adds --help commands to the psc-ide executables (@kRITZCREEK)
• Catches EOF exceptions thrown in acceptCommand (@kRITZCREEK)

Other

• Switched to Trusty distribution for Travis (@garyb)
• @kRITZCREEK and @faineance worked on refactoring the compiler.
• The optparse-applicative dependency was updated to >= 0.12.1 (@stevejb71)
• The bower-json dependency was bumped (@hdgarrood)
• Better error message for psc-publish tests (@kRITZCREEK)
• Use generic Literal in the AST (@garyb)

Breaking Changes

None

Enhancements

• psc-ide is now distributed with the compiler! (@kRITZCREEK)

  The psc-ide-server and psc-ide-client executables are now maintained and
  distributed alongside the compiler. This will ensure that the externs file
  format used by psc-ide-server is kept in sync with changes in the compiler.

• Source maps (@nwolverson)

  Source maps can be generated using the --source-maps flag. See the
  example repository for a full demonstration of source maps using Webpack.

• Operator aliases for data constructors (@garyb)

  Aliases can now be defined for data constructors. For example:

  data List a = Nil | Cons a (List a)
  
  infixr 6 Cons as :
  

  Here, the : operator can be used as a function to replace the Cons constructor,
  and also in binders.

• Eq and Ord deriving (@paf31)

  Eq and Ord instances can now be derived, using the derive instance syntax:

  derive instance eqList  :: (Eq  a) => Eq  (List a)
  derive instance ordList :: (Ord a) => Ord (List a)
  

• Types are now inferred in psc-docs and psc-publish (@hdgarrood)

  If type annotations are missing in source files, they will be inferred by
  psc-docs and psc-publish before documentation generation.

• Initial version of new syntax for operator sections (#1846, @paf31)

  Operator sections can now be written using underscores. For example:

  decrementAll :: Array Int -> Array Int
  decrementAll = map (_ - 1)
  

  which is equivalent to:

  decrementAll :: Array Int -> Array Int
  decrementAll = map (\x -> x - 1)
  

Bug Fixes

• Allow one open import without warning (@garyb)

  Warnings for open imports were a pain point for some users after the 0.8 release.
  This change allows a single open import without a warning. This is still safe
  in the presence of dependency updates, and does not lead to ambiguity for editor
  plugins searching for declaration sites.

Other

• @phadej has updated the Stack build to use the latest LTS and nightly builds.
• @izgzhen has refactored the PSCi code to be more readable.
• @hdgarrood has refactored the test suite.

You are recommended to use v0.8.2 instead.

A massive thanks to everyone involved in this release!

Breaking Changes

None, but there are lots of new warnings related to upcoming breaking changes in 0.9:

• Operators as aliases will become mandatory, and regular operators (as functions) will now generate warnings.
• Non-exhaustive functions will get a Partial constraint in 0.9, so the exhaustivity checker will now attempt to generate warnings by looking for Partial constraints in scope.
• The qualified import syntax has been deprecated.
• Class imports will use the new class syntax in 0.9 and the alternative syntax is deprecated.

Enhancements

• Add native Partial constraint (@garyb)
• Reduce backtracking in parser to hopefully improve quality of parsing error messages (@paf31)
• Drop requirement to parenthesize single constraints in instance contexts (@garyb)
• Case expressions can now match multiple values (@natefaubion)
• Add operator aliases (@garyb)
• Show identifiers correctly in ctags (@nwolverson)
• Fix #1523, add --json-errors flag for editor integrations (@paf31)
• Error and warning corrections are now available to editors via --json-errors (@nwolverson)
• Check integer values are within range in codegen (@garyb)
• Support for unicode operators (@paf31)
• The parser now supports unicode symbols for forall and function arrows (@DavidLindbom)
• Module Imports
  • Use class keyword for class references in imports (@garyb)
  • Type imports no longer require () (@garyb)
  • Allow import hiding with qualified imports (@garyb)
  • Naming conflicts are now resolved at the use site (@garyb)
• Error Messages
  • Fix #1662, display extra type info in instance errors (@paf31)
  • Add information about skolem constants to type errors (@paf31)
  • Sort rows in unification errors (@paf31)
• Warnings
  • Warn on unspecified imports (@garyb)
  • Warn when import X hiding (..) imports nothing (@garyb)
  • Warn on duplicate imports and exports (@garyb)
  • Warn about unused class imports (@garyb)

Bug Fixes

• Renamer updates, fixes naming bug in some unlikely situations (@garyb)
• Fix #1645, implement new indentation rules for types to avoid very wide errors (@paf31)
• Fix "resource exhausted" issue on MacOS (@mgmeier)
• Fix #1664, check kind before expanding wildcards. (@paf31)
• Fix up shadowed module names in JS codegen (@garyb)
• Fix #1185, fix #1369, add everythingWithScope traversal to correct some scoping issues. (@paf31)
• Fix two cases where errors were missing context (@garyb)
• Fix #1636, instantiate polytypes fully, even under constraints. (@paf31)
• Fix missing data constructors in re-exports (@garyb)
• Fix codegen error with instance for re-exported class (@garyb)
• Fix #1479, encode .js files as UTF8. (@paf31)
• Fix a bug related to redundancy checking in cases (#1853, @nicodelpiano)
• Fix a TCO/composition inlining bug (@garyb, @hdgarrood)
• Fix renaming for nested constructor binders (#1839, @sharkdp)
• Fix generic deriving bug with >1 type argument (@hdgarrood)
• Fix generate fresh binder names unless all names in case are equal (#1825, @paf31)
• Fix external require expressions when minifying (#1794, @paf31)
• Rename foreign argument to fix compiling issue (@anttih)
• Allow use of bottom integer (@garyb)

Other

• Fix #1700, remove warnings for syntactic features removed in 0.7.0 (@paf31)
• Fix psc-publish test (@passy)
• Relax rules for docs comments (#1820, @hdgarrood)
• Qualified name lookup is now supported in PSCi (#974, @soupi)
• https://github.com and [email protected] URLs are now allowed by psc-publish (@passy, @codedmart)
• Docs are now generated for module re-exports (@hdgarrood)
• Use friendly module name in psc-docs error (@nwolverson)
• Distinguish between the different ProperNames (@garyb)
• Warn about unspecified constructors in type imports (@garyb)
• Fix warning about values missing from virtual modules (@garyb)

Fixes a bug in generic deriving.

See the release notes for 0.7.6.

Thanks once again to everyone involved in this release!

This release includes some updates to generic deriving which require updating to the latest version of purescript-generics.

Features

• Field puns, fix #921 (@balajirrao)

  It is now possble to construct objects by using values in scope with the same name as the field labels. For example, the expression { foo, bar } is equivalent to { foo: foo, bar: bar }. Patterns desugar in the same way.

Enhancements

• Modules are now parsed in parallel (@paf31)
• Use Types.Proxy.Proxy instead of Data.Generic.Proxy. This fixes #1573 (@tfausak)
• Update generic deriving for latest purescript-generics changes (@paf31)
• New import warnings - unused data constructors, unused imports (@nwolverson)
• psc-publish: only warn on dirty working tree on dry runs (@hdgarrood)
• Add more information to psci :browse command (@soupi)
• Add support for --require-path option to psc-bundle (@natefaubion)
• Improved error reporting in psc-publish (@hdgarrood)
• Reduce noise in instance declarations in documentation (@hdgarrood)

Bug Fixes

• New approach to unification, fixing some loops in the type checker (@paf31)
• Fix #1632, instantiate type variables in anyProxy calls in generic instances (@paf31)
• Fix warnings for unqualified implicit imports (@nwolverson)
• Fix #1596, don't show type checker warnings in the event of an error (@paf31)
• Fix #1602, improvements around code generation of string literals (@michaelficarra)
• Fix #1090, allow accessors in operator sections (@paf31)
• Fix #1590, limit depth of pretty-printed expressions (@paf31)
• Fix #1591, use the 'negate' in scope (@paf31)
• Fix #1335, track scoped type variables when skolemizing (@paf31)
• Fix #1175, check types inside where clauses inside instances (@paf31)
• Some refactoring (@phadej)
• Fixed some error messages (@zudov)

Deployment

• Use base-compat to reduce the need for CPP (@phadej)
• Write license-generator in Haskell (@phadej)
• Add GHC 7.10.3 to CI build matrix (@phadej)

Bug Fixes

• #1072, #1130, #1578, #1577, #1582

Fixes a build issue with GHC versions < 7.10. Functionally equivalent to v0.7.5.1.

Bug Fixes

• Fix #1169, #1315, #1534, #1543, #1548, #1551, #1557, #1570
• Fix memory leak caused by WriterT (#1297) by @paf31
• Display hints after main error (#1563) by @paf31
• Friendlier errors by @paf31
• Documentation fixes by @nwolverson
• Haddock fixes by @trofi

A big thank you to everyone who was involved in this release, from filing issues, through fixing bugs to testing patches.

The main focus areas for this release, as part of the 0.8 milestone, were error messages and performance.

Breaking Changes

None!

Enhancements

• Pretty printing of types and expressions in errors was improved (@paf31)

• Externs files are now saved as JSON (@paf31)

• Support for parallel builds has been added (@paf31)
  Builds will now use multiple cores by default, but the number of capabilities can be modified by passing the -N option to the GHC runtime:

  psc <input files> +RTS -N8
  

• Binders can now be given type annotations (@5outh)

  For example:

  example = do
    n :: Int <- get
    put (n + 1)
  

  This can be useful when disambiguating types.

• There is a new warning for missing type signatures on top-level declarations (@paf31)

• There are new warnings for shadowed and unused type variables (@garyb)

• Contextual information in warnings was improved (@garyb)

• The qualified keyword is now optional when importing modules qualified (@michaelficarra)

• @zudov changed the behavior of PSCi on CTRL+C/D to match GHCi and other REPLs.

• A bug in row unification was fixed (#1310, @paf31)

• Constrained types can now be defined without a forall keyword. This is useful in some nullary type class and rank-N scenarios. (@paf31)

Bug Fixes

• @garyb added some additional checks for transitive module exports.
• Type synonyms are now expanded more eagerly to avoid some error cases in the type checker (@paf31)
• Better support for multi-byte UTF-8 characters (@senju)
• A check has been added to the exhaustivity checker to avoid exponential blowup (@paf31)
• Empty case statements are no longer syntactically valid (@zudov)

Other

• @aspidites fixed all compiler warnings in the core libraries.
• @zudov and @phadej have made improvements to the Stack distribution of the compiler, and the Stackage builds.
• @garyb has added a warning for operators in type classes, since they will be disallowed before 0.8.

This patch release fixes two bugs related to the new instance resolution algorithm and overlapping instances:

• psci would not work due to overlaps in the PSCI.Support module
• free would not build due to its dependency on inject

The solution for now is to make overlapping instances into a warning (instead of an error) at the site of their use.

Later we might revisit this decision and allow the user to express classes like Inject which are necessarily overlapping.