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diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 9e93fd7..36bb71c 100644
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -241,6 +241,14 @@ documentation describes all the libraries that come with GHC.
+
+
+ Enables overloaded string literals (see ).
+
+
+
+ Enables lexically-scoped type variables (see .GHC comes with a large collection of libraries arranged
- hierarchically; see the accompanying library documentation.
- There is an ongoing project to create and maintain a stable set
- of core libraries used by several Haskell
- compilers, and the libraries that GHC comes with represent the
- current status of that project. For more details, see Haskell
- Libraries.
-
+ hierarchically; see the accompanying library
+ documentation. More libraries to install are available
+ from HackageDB.
@@ -956,7 +961,7 @@ a data type with no constructors. For example:
Syntactically, the declaration lacks the "= constrs" part. The
type can be parameterised over types of any kind, but if the kind is
not * then an explicit kind annotation must be used
-(see ).
+(see ).
Such data types have only one value, namely bottom.
Nevertheless, they can be useful when defining "phantom types".
@@ -1218,7 +1223,7 @@ that collection of packages in a uniform manner. You can express
quite a bit of object-oriented-like programming this way.
-
+Why existential?
@@ -1241,9 +1246,9 @@ But Haskell programmers can safely think of the ordinary
adding a new existential quantification construct.
-
+
-
+Type classes
@@ -1303,9 +1308,9 @@ Notice the way that the syntax fits smoothly with that used for
universal quantification earlier.
-
+
-
+Record Constructors
@@ -1322,7 +1327,7 @@ data Counter a = forall self. NewCounter
Here tag is a public field, with a well-typed selector
function tag :: Counter a -> a. The self
type is hidden from the outside; any attempt to apply _this,
-_inc or _output as functions will raise a
+_inc or _display as functions will raise a
compile-time error. In other words, GHC defines a record selector function
only for fields whose type does not mention the existentially-quantified variables.
(This example used an underscore in the fields for which record selectors
@@ -1368,10 +1373,10 @@ setTag obj t = obj{ tag = t }
-
+
-
+Restrictions
@@ -1522,7 +1527,7 @@ declarations. Define your own instances!
-
+
@@ -2538,7 +2543,7 @@ the context and head of the instance declaration can each consist of arbitrary
following rules:
-For each assertion in the context:
+The Paterson Conditions: for each assertion in the context
No type variable has more occurrences in the assertion than in the headThe assertion has fewer constructors and variables (taken together
@@ -2546,7 +2551,7 @@ For each assertion in the context:
-The coverage condition. For each functional dependency,
+The Coverage Condition. For each functional dependency,
tvsleft->tvsright, of the class,
every type variable in
@@ -2558,11 +2563,15 @@ corresponding type in the instance declaration.
These restrictions ensure that context reduction terminates: each reduction
step makes the problem smaller by at least one
-constructor. For example, the following would make the type checker
-loop if it wasn't excluded:
-
- instance C a => C a where ...
-
+constructor. Both the Paterson Conditions and the Coverage Condition are lifted
+if you give the
+flag ().
+You can find lots of background material about the reason for these
+restrictions in the paper
+Understanding functional dependencies via Constraint Handling Rules.
+
+
For example, these are OK:
instance C Int [a] -- Multiple parameters
@@ -2614,11 +2623,6 @@ something more specific does not:
op = ... -- Default
-You can find lots of background material about the reason for these
-restrictions in the paper
-Understanding functional dependencies via Constraint Handling Rules.
-
@@ -2683,8 +2687,8 @@ makes instance inference go into a loop, because it requires the constraint
Nevertheless, GHC allows you to experiment with more liberal rules. If you use
the experimental flag
-fallow-undecidable-instances
-option, you can use arbitrary
-types in both an instance context and instance head. Termination is ensured by having a
+option, both the Paterson Conditions and the Coverage Condition
+(described in ) are lifted. Termination is ensured by having a
fixed-depth recursion stack. If you exceed the stack depth you get a
sort of backtrace, and the opportunity to increase the stack depth
with N.
@@ -3364,7 +3368,7 @@ and you'd be right. That is why they are an experimental feature.
================ END OF Linear Implicit Parameters commented out -->
-
+Explicitly-kinded quantification
@@ -3484,7 +3488,6 @@ including an operational type class context, is legal:
On the left or right (see f4, for example)
of a function arrow
- On the right of a function arrow (see ) As the argument of a constructor, or type of a field, in a data type declaration. For
example, any of the f1,f2,f3,g1,g2 above would be valid
field type signatures.
@@ -4056,6 +4059,81 @@ pattern binding must have the same context. For example, this is fine:
+
+Overloaded string literals
+
+
+
+GHC supports overloaded string literals. Normally a
+string literal has type String, but with overloaded string
+literals enabled (with -foverloaded-strings)
+ a string literal has type (IsString a) => a.
+
+
+This means that the usual string syntax can be used, e.g., for packed strings
+and other variations of string like types. String literals behave very much
+like integer literals, i.e., they can be used in both expressions and patterns.
+If used in a pattern the literal with be replaced by an equality test, in the same
+way as an integer literal is.
+
+
+The class IsString is defined as:
+
+class IsString a where
+ fromString :: String -> a
+
+The only predefined instance is the obvious one to make strings work as usual:
+
+instance IsString [Char] where
+ fromString cs = cs
+
+The class IsString is not in scope by default. If you want to mention
+it explicitly (for exmaple, to give an instance declaration for it), you can import it
+from module GHC.Exts.
+
+
+Haskell's defaulting mechanism is extended to cover string literals, when is specified.
+Specifically:
+
+
+Each type in a default declaration must be an
+instance of Numor of IsString.
+
+
+
+The standard defaulting rule (Haskell Report, Section 4.3.4)
+is extended thus: defaulting applies when all the unresolved constraints involve standard classes
+orIsString; and at least one is a numeric class
+orIsString.
+
+
+
+
+A small example:
+
+module Main where
+
+import GHC.Exts( IsString(..) )
+
+newtype MyString = MyString String deriving (Eq, Show)
+instance IsString MyString where
+ fromString = MyString
+
+greet :: MyString -> MyString
+greet "hello" = "world"
+greet other = other
+
+main = do
+ print $ greet "hello"
+ print $ greet "fool"
+
+
+
+Note that deriving Eq is necessary for the pattern matching
+to work since it gets translated into an equality comparison.
+
+
+
@@ -4167,6 +4245,14 @@ Tim Sheard is going to expand it.)
(It would make sense to do so, but it's hard to implement.)
+
+ Furthermore, you can only run a function at compile time if it is imported
+ from another module that is not part of a mutually-recursive group of modules
+ that includes the module currently being compiled. For example, when compiling module A,
+ you can only run Template Haskell functions imported from B if B does not import A (directly or indirectly).
+ The reason should be clear: to run B we must compile and run A, but we are currently type-checking A.
+
+
The flag -ddump-splices shows the expansion of all top-level splices as they happen.
@@ -4786,7 +4872,7 @@ Because the preprocessor targets Haskell (rather than Core),
-
+Bang patterns
Bang patterns
@@ -4801,7 +4887,7 @@ than the material below.
Bang patterns are enabled by the flag .
-
+Informal description of bang patterns
@@ -4856,7 +4942,7 @@ is part of the syntax of let bindings.
-
+Syntax and semantics
@@ -4930,7 +5016,7 @@ a module.
-
+Assertions
Assertions