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diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 8a21368..0a9c7e8 100644
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -116,11 +116,22 @@ documentation describes all the libraries that come with GHC.
- :
-
+ ,:
+
+
+ These two flags control how generalisation is done.
+ See .
+
+
+
+
+
+
+ :
+
- Switch off the Haskell 98 monomorphism restriction.
+ Use GHCi's extended default rules in a regular module ().
Independent of the
flag.
@@ -140,7 +151,7 @@ documentation describes all the libraries that come with GHC.
-
+
@@ -617,7 +628,7 @@ clunky env var1 var1 = case lookup env var1 of
Nothing -> fail
Just val2 -> val1 + val2
where
- fail = val1 + val2
+ fail = var1 + var2
@@ -2022,6 +2033,11 @@ 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.
+
@@ -2090,7 +2106,7 @@ option, you can use arbitrary
types in both an instance context and instance head. 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.
+with N.
@@ -2107,7 +2123,9 @@ can be modified by two flags:
and
-fallow-incoherent-instances
-, as this section discusses.
+, as this section discusses. Both these
+flags are dynamic flags, and can be set on a per-module basis, using
+an OPTIONS_GHC pragma if desired ().
When GHC tries to resolve, say, the constraint C Int Bool,
it tries to match every instance declaration against the
@@ -2464,7 +2482,7 @@ function that called it. For example, our sort function might
to pick out the least value in a list:
least :: (?cmp :: a -> a -> Bool) => [a] -> a
- least xs = fst (sort xs)
+ least xs = head (sort xs)
Without lifting a finger, the ?cmp parameter is
propagated to become a parameter of least as well. With explicit
@@ -2624,6 +2642,11 @@ inner binding of ?x, so (f 9) will return
+
+
Explicitly-kinded quantification
@@ -4236,7 +4261,46 @@ Hello
+
+
+Using Template Haskell with Profiling
+profilingwith Template Haskell
+Template Haskell relies on GHC's built-in bytecode compiler and
+interpreter to run the splice expressions. The bytecode interpreter
+runs the compiled expression on top of the same runtime on which GHC
+itself is running; this means that the compiled code referred to by
+the interpreted expression must be compatible with this runtime, and
+in particular this means that object code that is compiled for
+profiling cannot be loaded and used by a splice
+expression, because profiled object code is only compatible with the
+profiling version of the runtime.
+
+This causes difficulties if you have a multi-module program
+containing Template Haskell code and you need to compile it for
+profiling, because GHC cannot load the profiled object code and use it
+when executing the splices. Fortunately GHC provides a workaround.
+The basic idea is to compile the program twice:
+
+
+
+ Compile the program or library first the normal way, without
+ .
+
+
+ Then compile it again with , and
+ additionally use
+ to name the object files differentliy (you can choose any suffix
+ that isn't the normal object suffix here). GHC will automatically
+ load the object files built in the first step when executing splice
+ expressions. If you omit the flag when
+ building with and Template Haskell is used,
+ GHC will emit an error message.
+
+
+
+
@@ -4731,6 +4795,150 @@ Because the preprocessor targets Haskell (rather than Core),
+
+
+
+Bang patterns
+<indexterm><primary>Bang patterns</primary></indexterm>
+
+GHC supports an extension of pattern matching called bang
+patterns. Bang patterns are under consideration for Haskell Prime.
+The Haskell
+prime feature description contains more discussion and examples
+than the material below.
+
+
+Bang patterns are enabled by the flag .
+
+
+
+Informal description of bang patterns
+
+
+The main idea is to add a single new production to the syntax of patterns:
+
+ pat ::= !pat
+
+Matching an expression e against a pattern !p is done by first
+evaluating e (to WHNF) and then matching the result against p.
+Example:
+
+f1 !x = True
+
+This definition makes f1 is strict in x,
+whereas without the bang it would be lazy.
+Bang patterns can be nested of course:
+
+f2 (!x, y) = [x,y]
+
+Here, f2 is strict in x but not in
+y.
+A bang only really has an effect if it precedes a variable or wild-card pattern:
+
+f3 !(x,y) = [x,y]
+f4 (x,y) = [x,y]
+
+Here, f3 and f4 are identical; putting a bang before a pattern that
+forces evaluation anyway does nothing.
+
+Bang patterns work in case expressions too, of course:
+
+g5 x = let y = f x in body
+g6 x = case f x of { y -> body }
+g7 x = case f x of { !y -> body }
+
+The functions g5 and g6 mean exactly the same thing.
+But g7 evalutes (f x), binds y to the
+result, and then evaluates body.
+
+Bang patterns work in let and where
+definitions too. For example:
+
+let ![x,y] = e in b
+
+is a strict pattern: operationally, it evaluates e, matches
+it against the pattern [x,y], and then evaluates b
+The "!" should not be regarded as part of the pattern; after all,
+in a function argument ![x,y] means the
+same as [x,y]. Rather, the "!"
+is part of the syntax of let bindings.
+
+
+
+
+
+Syntax and semantics
+
+
+
+We add a single new production to the syntax of patterns:
+
+ pat ::= !pat
+
+There is one problem with syntactic ambiguity. Consider:
+
+f !x = 3
+
+Is this a definition of the infix function "(!)",
+or of the "f" with a bang pattern? GHC resolves this
+ambiguity inf favour of the latter. If you want to define
+(!) with bang-patterns enabled, you have to do so using
+prefix notation:
+
+(!) f x = 3
+
+The semantics of Haskell pattern matching is described in
+Section 3.17.2 of the Haskell Report. To this description add
+one extra item 10, saying:
+Matching
+the pattern !pat against a value v behaves as follows:
+if v is bottom, the match diverges
+ otherwise, pat is matched against
+ v
+
+
+Similarly, in Figure 4 of
+Section 3.17.3, add a new case (t):
+
+case v of { !pat -> e; _ -> e' }
+ = v `seq` case v of { pat -> e; _ -> e' }
+
+
+That leaves let expressions, whose translation is given in
+Section
+3.12
+of the Haskell Report.
+In the translation box, first apply
+the following transformation: for each pattern pi that is of
+form !qi = ei, transform it to (xi,!qi) = ((),ei), and and replace e0
+by (xi `seq` e0). Then, when none of the left-hand-side patterns
+have a bang at the top, apply the rules in the existing box.
+
+The effect of the let rule is to force complete matching of the pattern
+qi before evaluation of the body is begun. The bang is
+retained in the translated form in case qi is a variable,
+thus:
+
+ let !y = f x in b
+
+
+
+
+The let-binding can be recursive. However, it is much more common for
+the let-binding to be non-recursive, in which case the following law holds:
+(let !p = rhs in body)
+ is equivalent to
+(case rhs of !p -> body)
+
+
+A pattern with a bang at the outermost level is not allowed at the top level of
+a module.
+
+
+
+
@@ -5339,7 +5547,9 @@ The programmer can specify rewrite rules as part of the source program
(in a pragma). GHC applies these rewrite rules wherever it can, provided (a)
the flag () is on,
and (b) the flag
-() is not specified.
+() is not specified, and (c) the
+ ()
+flag is active.
@@ -6029,7 +6239,7 @@ shortcoming is something that could be fixed, with some kind of pragma.)
-The <literal>inline</literal> function
+The <literal>lazy</literal> function
The lazy function restrains strictness analysis a little:
@@ -6054,6 +6264,21 @@ look strict in y which would defeat the whole
purpose of par.
+
+The <literal>unsafeCoerce#</literal> function
+
+The function unsafeCoerce# allows you to side-step the
+typechecker entirely. It has type
+
+ unsafeCoerce# :: a -> b
+
+That is, it allows you to coerce any type into any other type. If you use this
+function, you had better get it right, otherwise segmentation faults await.
+It is generally used when you want to write a program that you know is
+well-typed, but where Haskell's type system is not expressive enough to prove
+that it is well typed.
+
+
@@ -6313,6 +6538,51 @@ Just to finish with, here's another example I rather like:
+
+Control over monomorphism
+
+GHC supports two flags that control the way in which generalisation is
+carried out at let and where bindings.
+
+
+
+Switching off the dreaded Monomorphism Restriction
+
+
+Haskell's monomorphism restriction (see
+Section
+4.5.5
+of the Haskell Report)
+can be completely switched off by
+.
+
+
+
+
+Monomorphic pattern bindings
+
+
+
+ As an experimental change, we are exploring the possibility of
+ making pattern bindings monomorphic; that is, not generalised at all.
+ A pattern binding is a binding whose LHS has no function arguments,
+ and is not a simple variable. For example:
+
+ f x = x -- Not a pattern binding
+ f = \x -> x -- Not a pattern binding
+ f :: Int -> Int = \x -> x -- Not a pattern binding
+
+ (g,h) = e -- A pattern binding
+ (f) = e -- A pattern binding
+ [x] = e -- A pattern binding
+
+Experimentally, GHC now makes pattern bindings monomorphic by
+default. Use to recover the
+standard behaviour.
+
+
+
+