</para>
</sect2>
+ <!-- ===================== View patterns =================== -->
+
+<sect2 id="view-patterns">
+<title>View patterns
+</title>
+
+<para>
+View patterns are enabled by the flag <literal>-XViewPatterns</literal>.
+More information and examples of view patterns can be found on the
+<ulink url="http://hackage.haskell.org/trac/ghc/wiki/ViewPatterns">Wiki
+page</ulink>.
+</para>
+
+<para>
+View patterns are somewhat like pattern guards that can be nested inside
+of other patterns. They are a convenient way of pattern-matching
+against values of abstract types. For example, in a programming language
+implementation, we might represent the syntax of the types of the
+language as follows:
+
+<programlisting>
+type Typ
+
+data TypView = Unit
+ | Arrow Typ Typ
+
+view :: Type -> TypeView
+
+-- additional operations for constructing Typ's ...
+</programlisting>
+
+The representation of Typ is held abstract, permitting implementations
+to use a fancy representation (e.g., hash-consing to managage sharing).
+
+Without view patterns, using this signature a little inconvenient:
+<programlisting>
+size :: Typ -> Integer
+size t = case view t of
+ Unit -> 1
+ Arrow t1 t2 -> size t1 + size t2
+</programlisting>
+
+It is necessary to iterate the case, rather than using an equational
+function definition. And the situation is even worse when the matching
+against <literal>t</literal> is buried deep inside another pattern.
+</para>
+
+<para>
+View patterns permit calling the view function inside the pattern and
+matching against the result:
+<programlisting>
+size (view -> Unit) = 1
+size (view -> Arrow t1 t2) = size t1 + size t2
+</programlisting>
+
+That is, we add a new form of pattern, written
+<replaceable>expression</replaceable> <literal>-></literal>
+<replaceable>pattern</replaceable> that means "apply the expression to
+whatever we're trying to match against, and then match the result of
+that application against the pattern". The expression can be any Haskell
+expression of function type, and view patterns can be used wherever
+patterns are used.
+</para>
+
+<para>
+The semantics of a pattern <literal>(</literal>
+<replaceable>exp</replaceable> <literal>-></literal>
+<replaceable>pat</replaceable> <literal>)</literal> are as follows:
+
+<itemizedlist>
+
+<listitem> Scoping:
+
+<para>The variables bound by the view pattern are the variables bound by
+<replaceable>pat</replaceable>.
+</para>
+
+<para>
+Any variables in <replaceable>exp</replaceable> are bound occurrences,
+but variables bound "to the left" in a pattern are in scope. This
+feature permits, for example, one argument to a function to be used in
+the view of another argument. For example, the function
+<literal>clunky</literal> from <xref linkend="pattern-guards" /> can be
+written using view patterns as follows:
+
+<programlisting>
+clunky env (lookup env -> Just val1) (lookup env -> Just val2) = val1 + val2
+...other equations for clunky...
+</programlisting>
+</para>
+
+<para>
+More precisely, the scoping rules are:
+<itemizedlist>
+<listitem>
+<para>
+In a single pattern, variables bound by patterns to the left of a view
+pattern expression are in scope. For example:
+<programlisting>
+example :: Maybe ((String -> Integer,Integer), String) -> Bool
+example Just ((f,_), f -> 4) = True
+</programlisting>
+
+Additionally, in function definitions, variables bound by matching earlier curried
+arguments may be used in view pattern expressions in later arguments:
+<programlisting>
+example :: (String -> Integer) -> String -> Bool
+example f (f -> 4) = True
+</programlisting>
+That is, the scoping is the same as it would be if the curried arguments
+were collected into a tuple.
+</para>
+</listitem>
+
+<listitem>
+<para>
+In mutually recursive bindings, such as <literal>let</literal>,
+<literal>where</literal>, or the top level, view patterns in one
+declaration may not mention variables bound by other declarations. That
+is, each declaration must be self-contained. For example, the following
+program is not allowed:
+<programlisting>
+let {(x -> y) = e1 ;
+ (y -> x) = e2 } in x
+</programlisting>
+
+(We may lift this
+restriction in the future; the only cost is that type checking patterns
+would get a little more complicated.)
+
+
+</para>
+</listitem>
+</itemizedlist>
+
+</para>
+</listitem>
+
+<listitem><para> Typing: If <replaceable>exp</replaceable> has type
+<replaceable>T1</replaceable> <literal>-></literal>
+<replaceable>T2</replaceable> and <replaceable>pat</replaceable> matches
+a <replaceable>T2</replaceable>, then the whole view pattern matches a
+<replaceable>T1</replaceable>.
+</para></listitem>
+
+<listitem><para> Matching: To the equations in Section 3.17.3 of the
+<ulink url="http://www.haskell.org/onlinereport/">Haskell 98
+Report</ulink>, add the following:
+<programlisting>
+case v of { (e -> p) -> e1 ; _ -> e2 }
+ =
+case (e v) of { p -> e1 ; _ -> e2 }
+</programlisting>
+That is, to match a variable <replaceable>v</replaceable> against a pattern
+<literal>(</literal> <replaceable>exp</replaceable>
+<literal>-></literal> <replaceable>pat</replaceable>
+<literal>)</literal>, evaluate <literal>(</literal>
+<replaceable>exp</replaceable> <replaceable> v</replaceable>
+<literal>)</literal> and match the result against
+<replaceable>pat</replaceable>.
+</para></listitem>
+
+<listitem><para> Efficiency: When the same view function is applied in
+multiple branches of a function definition or a case expression (e.g.,
+in <literal>size</literal> above), GHC makes an attempt to collect these
+applications into a single nested case expression, so that the view
+function is only applied once. Pattern compilation in GHC follows the
+matrix algorithm described in Chapter 4 of <ulink
+url="http://research.microsoft.com/~simonpj/Papers/slpj-book-1987/">The
+Implementation of Functional Programming Languages</ulink>. When the
+top rows of the first column of a matrix are all view patterns with the
+"same" expression, these patterns are transformed into a single nested
+case. This includes, for example, adjacent view patterns that line up
+in a tuple, as in
+<programlisting>
+f ((view -> A, p1), p2) = e1
+f ((view -> B, p3), p4) = e2
+</programlisting>
+</para>
+
+<para> The current notion of when two view pattern expressions are "the
+same" is very restricted: it is not even full syntactic equality.
+However, it does include variables, literals, applications, and tuples;
+e.g., two instances of <literal>view ("hi", "there")</literal> will be
+collected. However, the current implementation does not compare up to
+alpha-equivalence, so two instances of <literal>(x, view x ->
+y)</literal> will not be coalesced.
+</para>
+
+</listitem>
+
+</itemizedlist>
+</para>
+
+</sect2>
+
<!-- ===================== Recursive do-notation =================== -->
<sect2 id="mdo-notation">
</sect2>
+ <!-- ===================== REBINDABLE SYNTAX =================== -->
+
<sect2 id="rebindable-syntax">
<title>Rebindable syntax</title>
-
<para>GHC allows most kinds of built-in syntax to be rebound by
the user, to facilitate replacing the <literal>Prelude</literal>
with a home-grown version, for example.</para>
records from different modules that use the same field name.
</para>
</sect2>
+
+ <!-- ===================== Record puns =================== -->
+
+<sect2 id="record-puns">
+<title>Record puns
+</title>
+
+<para>
+Record puns are enabled by the flag <literal>-XRecordPuns</literal>.
+</para>
+
+<para>
+When using records, it is common to write a pattern that binds a
+variable with the same name as a record field, such as:
+
+<programlisting>
+data C = C {a :: Int}
+f (C {a = a}) = a
+</programlisting>
+</para>
+
+<para>
+Record punning permits the variable name to be elided, so one can simply
+write
+
+<programlisting>
+f (C {a}) = a
+</programlisting>
+
+to mean the same pattern as above. That is, in a record pattern, the
+pattern <literal>a</literal> expands into the pattern <literal>a =
+a</literal> for the same name <literal>a</literal>.
+</para>
+
+<para>
+Note that puns and other patterns can be mixed in the same record:
+<programlisting>
+data C = C {a :: Int, b :: Int}
+f (C {a, b = 4}) = a
+</programlisting>
+and that puns can be used wherever record patterns occur (e.g. in
+<literal>let</literal> bindings or at the top-level).
+</para>
+
+<para>
+Record punning can also be used in an expression, writing, for example,
+<programlisting>
+let a = 1 in C {a}
+</programlisting>
+instead of
+<programlisting>
+let a = 1 in C {a = a}
+</programlisting>
+
+Note that this expansion is purely syntactic, so the record pun
+expression refers to the nearest enclosing variable that is spelled the
+same as the field name.
+</para>
+
+</sect2>
+
+ <!-- ===================== Record wildcards =================== -->
+
+<sect2 id="record-wildcards">
+<title>Record wildcards
+</title>
+
+<para>
+Record wildcards are enabled by the flag <literal>-XRecordWildCards</literal>.
+</para>
+
+<para>
+For records with many fields, it can be tiresome to write out each field
+individually in a record pattern, as in
+<programlisting>
+data C = C {a :: Int, b :: Int, c :: Int, d :: Int}
+f (C {a = 1, b = b, c = c, d = d}) = b + c + d
+</programlisting>
+</para>
+
+<para>
+Record wildcard syntax permits a (<literal>..</literal>) in a record
+pattern, where each elided field <literal>f</literal> is replaced by the
+pattern <literal>f = f</literal>. For example, the above pattern can be
+written as
+<programlisting>
+f (C {a = 1, ..}) = b + c + d
+</programlisting>
+</para>
+
+<para>
+Note that wildcards can be mixed with other patterns, including puns
+(<xref linkend="record-puns"/>); for example, in a pattern <literal>C {a
+= 1, b, ..})</literal>. Additionally, record wildcards can be used
+wherever record patterns occur, including in <literal>let</literal>
+bindings and at the top-level. For example, the top-level binding
+<programlisting>
+C {a = 1, ..} = e
+</programlisting>
+defines <literal>b</literal>, <literal>c</literal>, and
+<literal>d</literal>.
+</para>
+
+<para>
+Record wildcards can also be used in expressions, writing, for example,
+
+<programlisting>
+let {a = 1; b = 2; c = 3; d = 4} in C {..}
+</programlisting>
+
+in place of
+
+<programlisting>
+let {a = 1; b = 2; c = 3; d = 4} in C {a=a, b=b, c=c, d=d}
+</programlisting>
+
+Note that this expansion is purely syntactic, so the record wildcard
+expression refers to the nearest enclosing variables that are spelled
+the same as the omitted field names.
+</para>
+
+</sect2>
+
+ <!-- ===================== Local fixity declarations =================== -->
+
+<sect2 id="local-fixity-declarations">
+<title>Local Fixity Declarations
+</title>
+
+<para>A careful reading of the Haskell 98 Report reveals that fixity
+declarations (<literal>infix</literal>, <literal>infixl</literal>, and
+<literal>infixr</literal>) are permitted to appear inside local bindings
+such those introduced by <literal>let</literal> and
+<literal>where</literal>. However, the Haskell Report does not specify
+the semantics of such bindings very precisely.
+</para>
+
+<para>In GHC, a fixity declaration may accompany a local binding:
+<programlisting>
+let f = ...
+ infixr 3 `f`
+in
+ ...
+</programlisting>
+and the fixity declaration applies wherever the binding is in scope.
+For example, in a <literal>let</literal>, it applies in the right-hand
+sides of other <literal>let</literal>-bindings and the body of the
+<literal>let</literal>C. Or, in recursive <literal>do</literal>
+expressions (<xref linkend="mdo-notation"/>), the local fixity
+declarations of aA <literal>let</literal> statement scope over other
+statements in the group, just as the bound name does.
+</para>
+
+Moreover, a local fixity declatation *must* accompany a local binding of
+that name: it is not possible to revise the fixity of name bound
+elsewhere, as in
+<programlisting>
+let infixr 9 $ in ...
+</programlisting>
+
+Because local fixity declarations are technically Haskell 98, no flag is
+necessary to enable them.
+</sect2>
+
</sect1>
generated by the call to <literal>elem</literal>, so that the type of
<literal>insert</literal> itself has no <literal>Eq</literal> constraint.
</para>
-<para>This behaviour contrasts with Haskell 98's peculiar treament of
+<para>This behaviour contrasts with Haskell 98's peculiar treatment of
contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report).
-In Haskell 98 the defintion
+In Haskell 98 the definition
<programlisting>
data Eq a => Set' a = MkSet' [a]
</programlisting>
<para>
At the moment, record updates are not yet possible with GADT-style declarations,
so support is limited to record construction, selection and pattern matching.
-For exmaple
+For example
<programlisting>
aPerson = Adult { name = "Fred", children = [] }
<listitem><para>
You cannot use a <literal>deriving</literal> clause for a GADT; only for
-an ordianary data type.
+an ordinary data type.
</para></listitem>
<listitem><para>
<para>
GHC takes a conservative position: it accepts the first two, but not the third. The rule is this:
each constraint in the inferred instance context must consist only of type variables,
-with no repititions.
+with no repetitions.
</para>
<para>
This rule is applied regardless of flags. If you want a more exotic context, you can write
deriving instance MonadState Int Foo
</programlisting>
GHC always treats the <emphasis>last</emphasis> parameter of the instance
-(<literal>Foo</literal> in this exmample) as the type whose instance is being derived.
+(<literal>Foo</literal> in this example) as the type whose instance is being derived.
</para>
</sect2>
The willingness to be overlapped or incoherent is a property of
the <emphasis>instance declaration</emphasis> itself, controlled by the
presence or otherwise of the <option>-XOverlappingInstances</option>
-and <option>-XIncoherentInstances</option> flags when that mdodule is
+and <option>-XIncoherentInstances</option> flags when that module is
being defined. Neither flag is required in a module that imports and uses the
instance declaration. Specifically, during the lookup process:
<itemizedlist>
fromString cs = cs
</programlisting>
The class <literal>IsString</literal> 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
+it explicitly (for example, to give an instance declaration for it), you can import it
from module <literal>GHC.Exts</literal>.
</para>
<para>
Boston, Jan 2000.
</para>
-<para>(Most of the following, stil rather incomplete, documentation is
+<para>(Most of the following, still rather incomplete, documentation is
due to Jeff Lewis.)</para>
<para>Implicit parameter support is enabled with the option
right-hand side, so the implicit parameter <literal>?acc</literal> is not
passed to the recursive call. In the latter case, because <literal>len_acc2</literal>
has a type signature, the recursive call is made to the
-<emphasis>polymoprhic</emphasis> version, which takes <literal>?acc</literal>
+<emphasis>polymorphic</emphasis> version, which takes <literal>?acc</literal>
as an implicit parameter. So we get the following results in GHCi:
<programlisting>
Prog> len1 "hello"
<itemizedlist>
<listitem> <para> '<literal>?x</literal>' and '<literal>%x</literal>'
are entirely distinct implicit parameters: you
- can use them together and they won't intefere with each other. </para>
+ can use them together and they won't interfere with each other. </para>
</listitem>
<listitem> <para> You can bind linear implicit parameters in 'with' clauses. </para> </listitem>
design.)</para></listitem>
<listitem><para>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
+(including one that contains free scoped type variables) denotes a
<emphasis>rigid</emphasis> type; that is, the type is fully known to the type
checker, and no inference is involved.</para></listitem>
<listitem><para>Lexical type variables may be alpha-renamed freely, without
</itemizedlist>
</para>
<para>
-In Haskell, a programmer-written type signature is implicitly quantifed over
+In Haskell, a programmer-written type signature is implicitly quantified over
its free type variables (<ulink
url="http://haskell.org/onlinereport/decls.html#sect4.1.2">Section
4.1.2</ulink>
g (x::a) = x
h ((x,y) :: (Int,Bool)) = (y,x)
</programlisting>
-In the case where all the type variables in the pattern type sigature are
+In the case where all the type variables in the pattern type signature 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.
</para>
<para>
If this seems a little odd, we think so too. But we must have
<emphasis>some</emphasis> way to bring such type variables into scope, else we
-could not name existentially-bound type variables in subequent type signatures.
+could not name existentially-bound type variables in subsequent type signatures.
</para>
<para>
This is (now) the <emphasis>only</emphasis> situation in which a pattern type
<literal>g</literal> is typechecked first, separately from that for
<literal>f</literal>,
because the reference to <literal>f</literal> in <literal>g</literal>'s right
-hand side is ingored by the dependency analysis. Then <literal>g</literal>'s
+hand side is ignored by the dependency analysis. Then <literal>g</literal>'s
type is generalised, to get
<programlisting>
g :: Ord a => a -> Bool
</programlisting>
-Now, the defintion for <literal>f</literal> is typechecked, with this type for
+Now, the definition for <literal>f</literal> is typechecked, with this type for
<literal>g</literal> in the type environment.
</para>
<para>Then compile it again with <option>-prof</option>, and
additionally use <option>-osuf
p_o</option><indexterm><primary><option>-osuf</option></primary></indexterm>
- to name the object files differentliy (you can choose any suffix
+ to name the object files differently (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 <option>-osuf</option> flag when
g7 x = case f x of { !y -> body }
</programlisting>
The functions <literal>g5</literal> and <literal>g6</literal> mean exactly the same thing.
-But <literal>g7</literal> evalutes <literal>(f x)</literal>, binds <literal>y</literal> to the
+But <literal>g7</literal> evaluates <literal>(f x)</literal>, binds <literal>y</literal> to the
result, and then evaluates <literal>body</literal>.
</para><para>
Bang patterns work in <literal>let</literal> and <literal>where</literal>
<para>When you compile any module that imports and uses any
of the specified entities, GHC will print the specified
message.</para>
- <para> You can only depecate entities declared at top level in the module
+ <para> You can only deprecate entities declared at top level in the module
being compiled, and you can only use unqualified names in the list of
entities being deprecated. A capitalised name, such as <literal>T</literal>
refers to <emphasis>either</emphasis> the type constructor <literal>T</literal>
<programlisting>
{-# SPECIALIZE f :: <type> #-}
</programlisting>
- is valid if and only if the defintion
+ is valid if and only if the definition
<programlisting>
f_spec :: <type>
f_spec = f
<para>A <literal>SPECIALIZE</literal> pragma can optionally be followed with a
<literal>INLINE</literal> or <literal>NOINLINE</literal> pragma, optionally
followed by a phase, as described in <xref linkend="inline-noinline-pragma"/>.
-The <literal>INLINE</literal> pragma affects the specialised verison of the
+The <literal>INLINE</literal> pragma affects the specialised version of the
function (only), and applies even if the function is recursive. The motivating
example is this:
<programlisting>
<listitem>
<para>
- The definition of (say) <function>build</function> in <filename>GHC/Base.lhs</filename> looks llike this:
+ The definition of (say) <function>build</function> in <filename>GHC/Base.lhs</filename> looks like this:
<programlisting>
build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
<sect1 id="special-ids">
<title>Special built-in functions</title>
-<para>GHC has a few built-in funcions with special behaviour. These
+<para>GHC has a few built-in functions with special behaviour. These
are now described in the module <ulink
url="../libraries/base/GHC-Prim.html"><literal>GHC.Prim</literal></ulink>
in the library documentation.</para>
op2 :: a -> Bool
op2 {| p :*: q |} (x :*: y) = False
</programlisting>
-(The reason for this restriction is that we gather all the equations for a particular type consructor
+(The reason for this restriction is that we gather all the equations for a particular type constructor
into a single generic instance declaration.)
</para>
</listitem>
inside a list.
</para>
<para>
-This restriction is an implementation restriction: we just havn't got around to
+This restriction is an implementation restriction: we just haven't got around to
implementing the necessary bidirectional maps over arbitrary type constructors.
It would be relatively easy to add specific type constructors, such as Maybe and list,
to the ones that are allowed.</para>
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