X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=f8ad5c1b0ac8a79f9ee96c6a4227e6abafcaa6e7;hp=6b41b39057bbdd4cc4f3b81ebfbc72af919610d6;hb=9da4639011348fb6c318e3cba4b08622f811d9c4;hpb=3eeeb57d95c6a218ce457f85e1f5b3511057101f
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 6b41b39..f8ad5c1 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.
- :
-
+ ,:
- Switch off the Haskell 98 monomorphism restriction.
+ These two flags control how generalisation is done.
+ See .
+
+
+
+
+
+
+ :
+
+
+
+ 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
@@ -894,6 +905,38 @@ fromInteger :: Integer -> Bool -> Bool
you should be all right.
+
+
+Postfix operators
+
+
+GHC allows a small extension to the syntax of left operator sections, which
+allows you to define postfix operators. The extension is this: the left section
+
+ (e !)
+
+is equivalent (from the point of view of both type checking and execution) to the expression
+
+ ((!) e)
+
+(for any expression e and operator (!).
+The strict Haskell 98 interpretation is that the section is equivalent to
+
+ (\y -> (!) e y)
+
+That is, the operator must be a function of two arguments. GHC allows it to
+take only one argument, and that in turn allows you to write the function
+postfix.
+
+Since this extension goes beyond Haskell 98, it should really be enabled
+by a flag; but in fact it is enabled all the time. (No Haskell 98 programs
+change their behaviour, of course.)
+
+The extension does not extend to the left-hand side of function
+definitions; you must define such a function in prefix form.
+
+
+
@@ -2022,6 +2065,11 @@ 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 +2138,7 @@ 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.
@@ -2178,8 +2226,20 @@ some other constraint. But if the instance declaration was compiled with
check for that declaration.
-All this makes it possible for a library author to design a library that relies on
-overlapping instances without the library client having to know.
+These rules make it possible for a library author to design a library that relies on
+overlapping instances without the library client having to know.
+
+
+If an instance declaration is compiled without
+,
+then that instance can never be overlapped. This could perhaps be
+inconvenient. Perhaps the rule should instead say that the
+overlapping instance declaration should be compiled in
+this way, rather than the overlapped one. Perhaps overlap
+at a usage site should be permitted regardless of how the instance declarations
+are compiled, if the flag is
+used at the usage site. (Mind you, the exact usage site can occasionally be
+hard to pin down.) We are interested to receive feedback on these points.
The flag implies the
flag, but not vice versa.
@@ -2466,7 +2526,7 @@ 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
@@ -2626,6 +2686,11 @@ inner binding of ?x, so (f 9) will return
+
+
Explicitly-kinded quantification
@@ -3163,228 +3230,103 @@ for rank-2 types.
-
+
+Impredicative polymorphism
+
+GHC supports impredicative polymorphism. This means
+that you can call a polymorphic function at a polymorphic type, and
+parameterise data structures over polymorphic types. For example:
+
+ f :: Maybe (forall a. [a] -> [a]) -> Maybe ([Int], [Char])
+ f (Just g) = Just (g [3], g "hello")
+ f Nothing = Nothing
+
+Notice here that the Maybe type is parameterised by the
+polymorphic type (forall a. [a] ->
+[a]).
+
+The technical details of this extension are described in the paper
+Boxy types:
+type inference for higher-rank types and impredicativity,
+which appeared at ICFP 2006.
+
+
-Scoped type variables
+<title>Lexically scoped type variables
-A lexically scoped type variable can be bound by:
-
-A declaration type signature ()
-A pattern type signature ()
-A result type signature ()
-
-For example:
+GHC supports lexically scoped type variables, without
+which some type signatures are simply impossible to write. For example:
-f (xs::[a]) = ys ++ ys
- where
- ys :: [a]
- ys = reverse xs
+f :: forall a. [a] -> [a]
+f xs = ys ++ ys
+ where
+ ys :: [a]
+ ys = reverse xs
-The pattern (xs::[a]) includes a type signature for xs.
-This brings the type variable a into scope; it scopes over
-all the patterns and right hand sides for this equation for f.
-In particular, it is in scope at the type signature for y.
-
-
-
-At ordinary type signatures, such as that for ys, any type variables
-mentioned in the type signature that are not in scope are
-implicitly universally quantified. (If there are no type variables in
-scope, all type variables mentioned in the signature are universally
-quantified, which is just as in Haskell 98.) In this case, since a
-is in scope, it is not universally quantified, so the type of ys is
-the same as that of xs. In Haskell 98 it is not possible to declare
+The type signature for f brings the type variable a into scope; it scopes over
+the entire definition of f.
+In particular, it is in scope at the type signature for ys.
+In Haskell 98 it is not possible to declare
a type for ys; a major benefit of scoped type variables is that
it becomes possible to do so.
-
-
-Scoped type variables are implemented in both GHC and Hugs. Where the
-implementations differ from the specification below, those differences
-are noted.
-
-
-
-So much for the basic idea. Here are the details.
+Lexically-scoped type variables are enabled by
+.
+Note: GHC 6.6 contains substantial changes to the way that scoped type
+variables work, compared to earlier releases. Read this section
+carefully!
-What a scoped type variable means
-
-A lexically-scoped type variable is simply
-the name for a type. The restriction it expresses is that all occurrences
-of the same name mean the same type. For example:
-
- f :: [Int] -> Int -> Int
- f (xs::[a]) (y::a) = (head xs + y) :: a
-
-The pattern type signatures on the left hand side of
-f express the fact that xs
-must be a list of things of some type a; and that y
-must have this same type. The type signature on the expression (head xs)
-specifies that this expression must have the same type a.
-There is no requirement that the type named by "a" is
-in fact a type variable. Indeed, in this case, the type named by "a" is
-Int. (This is a slight liberalisation from the original rather complex
-rules, which specified that a pattern-bound type variable should be universally quantified.)
-For example, all of these are legal:
-
-
- t (x::a) (y::a) = x+y*2
-
- f (x::a) (y::b) = [x,y] -- a unifies with b
-
- g (x::a) = x + 1::Int -- a unifies with Int
-
- h x = let k (y::a) = [x,y] -- a is free in the
- in k x -- environment
-
- k (x::a) True = ... -- a unifies with Int
- k (x::Int) False = ...
-
- w :: [b] -> [b]
- w (x::a) = x -- a unifies with [b]
-
-
-
-
-
-Scope and implicit quantification
-
-
+Overview
+The design follows the following principles
-
-
-
-All the type variables mentioned in a pattern,
-that are not already in scope,
-are brought into scope by the pattern. We describe this set as
-the type variables bound by the pattern.
-For example:
-
- f (x::a) = let g (y::(a,b)) = fst y
- in
- g (x,True)
-
-The pattern (x::a) brings the type variable
-a into scope, as well as the term
-variable x. The pattern (y::(a,b))
-contains an occurrence of the already-in-scope type variable a,
-and brings into scope the type variable b.
-
-
-
-
-
-The type variable(s) bound by the pattern have the same scope
-as the term variable(s) bound by the pattern. For example:
-
- let
- f (x::a) = <...rhs of f...>
- (p::b, q::b) = (1,2)
- in <...body of let...>
-
-Here, the type variable a scopes over the right hand side of f,
-just like x does; while the type variable b scopes over the
-body of the let, and all the other definitions in the let,
-just like p and q do.
-Indeed, the newly bound type variables also scope over any ordinary, separate
-type signatures in the let group.
-
-
-
-
-
-
-The type variables bound by the pattern may be
-mentioned in ordinary type signatures or pattern
-type signatures anywhere within their scope.
-
-
-
-
-
-
- In ordinary type signatures, any type variable mentioned in the
-signature that is in scope is not universally quantified.
-
-
-
-
-
-
-
- Ordinary type signatures do not bring any new type variables
-into scope (except in the type signature itself!). So this is illegal:
-
-
- f :: a -> a
- f x = x::a
-
-
-It's illegal because a is not in scope in the body of f,
-so the ordinary signature x::a is equivalent to x::forall a.a;
-and that is an incorrect typing.
-
+A scoped type variable stands for a type variable, and not for
+a type. (This is a change from GHC's earlier
+design.)
+Furthermore, distinct lexical type variables stand for distinct
+type variables. This means that every programmer-written type signature
+(includin one that contains free scoped type variables) denotes a
+rigid type; that is, the type is fully known to the type
+checker, and no inference is involved.
+Lexical type variables may be alpha-renamed freely, without
+changing the program.
+
-
-
-
-The pattern type signature is a monotype:
-
-
+A lexically scoped type variable can be bound by:
-
-A pattern type signature cannot contain any explicit forall quantification.
-
-
-
-The type variables bound by a pattern type signature can only be instantiated to monotypes,
-not to type schemes.
-
-
-
-There is no implicit universal quantification on pattern type signatures (in contrast to
-ordinary type signatures).
-
-
+A declaration type signature ()
+An expression type signature ()
+A pattern type signature ()
+Class and instance declarations ()
-
-
-
-
+
-
-The type variables in the head of a class or instance declaration
-scope over the methods defined in the where part. For example:
-
-
+In Haskell, a programmer-written type signature is implicitly quantifed over
+its free type variables (Section
+4.1.2
+of the Haskel Report).
+Lexically scoped type variables affect this implicit quantification rules
+as follows: any type variable that is in scope is not universally
+quantified. For example, if type variable a is in scope,
+then
- class C a where
- op :: [a] -> a
-
- op xs = let ys::[a]
- ys = reverse xs
- in
- head ys
+ (e :: a -> a) means (e :: a -> a)
+ (e :: b -> b) means (e :: forall b. b->b)
+ (e :: a -> b) means (e :: forall b. a->b)
-
-
-(Not implemented in Hugs yet, Dec 98).
-
-
-
-
+
Declaration type signaturesA declaration type signature that has explicit
@@ -3411,180 +3353,145 @@ quantification rules.
-
-Where a pattern type signature can occur
-
-
-A pattern type signature can occur in any pattern. For example:
-
-
-
-
-A pattern type signature can be on an arbitrary sub-pattern, not
-just on a variable:
-
-
-
- f ((x,y)::(a,b)) = (y,x) :: (b,a)
-
-
-
-
-
-
-
-
- Pattern type signatures, including the result part, can be used
-in lambda abstractions:
-
-
- (\ (x::a, y) :: a -> x)
-
-
-
-
-
-
- Pattern type signatures, including the result part, can be used
-in case expressions:
+
+Expression type signatures
+An expression type signature that has explicit
+quantification (using forall) brings into scope the
+explicitly-quantified
+type variables, in the annotated expression. For example:
- case e of { ((x::a, y) :: (a,b)) -> x }
+ f = runST ( (op >>= \(x :: STRef s Int) -> g x) :: forall s. ST s Bool )
-
-Note that the -> symbol in a case alternative
-leads to difficulties when parsing a type signature in the pattern: in
-the absence of the extra parentheses in the example above, the parser
-would try to interpret the -> as a function
-arrow and give a parse error later.
-
+Here, the type signature forall a. ST s Bool brings the
+type variable s into scope, in the annotated expression
+(op >>= \(x :: STRef s Int) -> g x).
-
+
-
+
+Pattern type signatures
-To avoid ambiguity, the type after the “::” in a result
-pattern signature on a lambda or case must be atomic (i.e. a single
-token or a parenthesised type of some sort). To see why,
-consider how one would parse this:
-
-
+A type signature may occur in any pattern; this is a pattern type
+signature.
+For example:
- \ x :: a -> b -> x
+ -- f and g assume that 'a' is already in scope
+ f = \(x::Int, y::a) -> x
+ g (x::a) = x
+ h ((x,y) :: (Int,Bool)) = (y,x)
-
-
+In the case where all the type variables in the pattern type sigature are
+already in scope (i.e. bound by the enclosing context), matters are simple: the
+signature simply constrains the type of the pattern in the obvious way.
-
-
-
-
- Pattern type signatures can bind existential type variables.
-For example:
-
-
+There is only one situation in which you can write a pattern type signature that
+mentions a type variable that is not already in scope, namely in pattern match
+of an existential data constructor. For example:
data T = forall a. MkT [a]
- f :: T -> T
- f (MkT [t::a]) = MkT t3
+ k :: T -> T
+ k (MkT [t::a]) = MkT t3
where
t3::[a] = [t,t,t]
-
-
+Here, the pattern type signature (t::a) mentions a lexical type
+variable that is not already in scope. Indeed, it cannot already be in scope,
+because it is bound by the pattern match. GHC's rule is that in this situation
+(and only then), a pattern type signature can mention a type variable that is
+not already in scope; the effect is to bring it into scope, standing for the
+existentially-bound type variable.
-
-
-
-
-
-Pattern type signatures
-can be used in pattern bindings:
-
-
- f x = let (y, z::a) = x in ...
- f1 x = let (y, z::Int) = x in ...
- f2 (x::(Int,a)) = let (y, z::a) = x in ...
- f3 :: (b->b) = \x -> x
-
-
-In all such cases, the binding is not generalised over the pattern-bound
-type variables. Thus f3 is monomorphic; f3
-has type b -> b for some type b,
-and notforall b. b -> b.
-In contrast, the binding
-
- f4 :: b->b
- f4 = \x -> x
-
-makes a polymorphic function, but b is not in scope anywhere
-in f4's scope.
-
+If this seems a little odd, we think so too. But we must have
+some way to bring such type variables into scope, else we
+could not name existentially-bound type variables in subequent type signatures.
-
-
+
+This is (now) the only situation in which a pattern type
+signature is allowed to mention a lexical variable that is not already in
+scope.
+For example, both f and g would be
+illegal if a was not already in scope.
-Pattern type signatures are completely orthogonal to ordinary, separate
-type signatures. The two can be used independently or together.
+
+
+
+
+Class and instance declarations
-Result type signatures are not yet implemented in Hugs.
-
+The type variables in the head of a class or instance declaration
+scope over the methods defined in the where part. For example:
+
+
+
+ class C a where
+ op :: [a] -> a
+
+ op xs = let ys::[a]
+ ys = reverse xs
+ in
+ head ys
+
+
@@ -3751,16 +3658,19 @@ declaration (after expansion of any type synonyms)
where
- The type t is an arbitrary type
+ The ci are partial applications of
+ classes of the form C t1'...tj', where the arity of C
+ is exactly j+1. That is, C lacks exactly one type argument.
- The vk+1...vn are type variables which do not occur in
- t, and
+ The k is chosen so that ci (T v1...vk) is well-kinded.
- The ci are partial applications of
- classes of the form C t1'...tj', where the arity of C
- is exactly j+1. That is, C lacks exactly one type argument.
+ The type t is an arbitrary type.
+
+
+ The type variables vk+1...vn do not occur in t,
+ nor in the ci, and
None of the ci is Read, Show,
@@ -3773,13 +3683,8 @@ where
Then, for each ci, the derived instance
declaration is:
- instance ci (t vk+1...v) => ci (T v1...vp)
+ instance ci t => ci (T v1...vk)
-where p is chosen so that T v1...vp is of the
-right kind for the last parameter of class Ci.
-
-
-
As an example which does not work, consider
newtype NonMonad m s = NonMonad (State s m s) deriving Monad
@@ -3891,9 +3796,9 @@ pattern binding must have the same context. For example, this is fine:
-Generalised Algebraic Data Types
+Generalised Algebraic Data Types (GADTs)
-Generalised Algebraic Data Types (GADTs) generalise ordinary algebraic data types by allowing you
+Generalised Algebraic Data Types generalise ordinary algebraic data types by allowing you
to give the type signatures of constructors explicitly. For example:
data Term a where
@@ -3914,9 +3819,20 @@ for these Terms:
eval (If b e1 e2) = if eval b then eval e1 else eval e2
eval (Pair e1 e2) = (eval e1, eval e2)
-These and many other examples are given in papers by Hongwei Xi, and Tim Sheard.
+These and many other examples are given in papers by Hongwei Xi, and
+Tim Sheard. There is a longer introduction
+on the wiki,
+and Ralf Hinze's
+Fun with phantom types also has a number of examples. Note that papers
+may use different notation to that implemented in GHC.
- The extensions to GHC are these:
+
+The rest of this section outlines the extensions to GHC that support GADTs.
+It is far from comprehensive, but the design closely follows that described in
+the paper Simple
+unification-based type inference for GADTs,
+which appeared in ICFP 2006.
Data type declarations have a 'where' form, as exemplified above. The type signature of
@@ -4008,8 +3924,8 @@ declaration, but only if the data type could also have been declared in
Haskell-98 syntax. For example, these two declarations are equivalent
data Maybe1 a where {
- Nothing1 :: Maybe a ;
- Just1 :: a -> Maybe a
+ Nothing1 :: Maybe1 a ;
+ Just1 :: a -> Maybe1 a
} deriving( Eq, Ord )
data Maybe2 a = Nothing2 | Just2 a
@@ -4059,19 +3975,26 @@ the result type of the case expression. Hence the addition <
Template Haskell
-Template Haskell allows you to do compile-time meta-programming in Haskell. There is a "home page" for
-Template Haskell at
-http://www.haskell.org/th/, while
-the background to
+Template Haskell allows you to do compile-time meta-programming in
+Haskell.
+The background to
the main technical innovations is discussed in "
Template Meta-programming for Haskell" (Proc Haskell Workshop 2002).
-The details of the Template Haskell design are still in flux. Make sure you
-consult the online library reference material
+
+
+There is a Wiki page about
+Template Haskell at
+http://www.haskell.org/th/, and that is the best place to look for
+further details.
+You may also
+consult the online
+Haskell library reference material
(search for the type ExpQ).
[Temporary: many changes to the original design are described in
"http://research.microsoft.com/~simonpj/tmp/notes2.ps".
-Not all of these changes are in GHC 6.2.]
+Not all of these changes are in GHC 6.6.]
The first example from that paper is set out below as a worked example to help get you started.
@@ -4772,6 +4695,150 @@ 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.
+
+
+
+
@@ -6016,7 +6083,7 @@ r)
GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
r) ->
tpl2})
- (%note "foo"
+ (%note "bar"
eta);
@@ -6072,7 +6139,7 @@ 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:
@@ -6097,15 +6164,27 @@ 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.
+
+Generic classes
- (Note: support for generic classes is currently broken in
- GHC 5.02).
-
The ideas behind this extension are described in detail in "Derivable type classes",
Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp94-105.
@@ -6356,6 +6435,51 @@ 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.
+
+
+
+