The language extension XMagicHash
allows "#" as a
postfix modifier to identifiers. Thus, "x#" is a valid variable, and "T#" is
a valid type constructor or data constructor.
The hash sign does not change sematics at all. We tend to use variable
names ending in "#" for unboxed values or types (e.g. Int#
),
but there is no requirement to do so; they are just plain ordinary variables.
Nor does the XMagicHash
extension bring anything into scope.
For example, to bring Int#
into scope you must
import GHC.Prim
(see Section 7.2, “Unboxed types and primitive operations”);
the XMagicHash
extension
then allows you to refer to the Int#
that is now in scope.
The XMagicHash
also enables some new forms of literals (see Section 7.2.1, “Unboxed types
”):
'x'#
has type Char#
"foo"#
has type Addr#
3#
has type Int#
. In general,
any Haskell 98 integer lexeme followed by a #
is an Int#
literal, e.g.
0x3A#
as well as 32#
3##
has type Word#
. In general,
any nonnegative Haskell 98 integer lexeme followed by ##
is a Word#
.
3.2#
has type Float#
.
3.2##
has type Double#
A new syntax for referencing qualified operators is
planned to be introduced by Haskell', and is enabled in GHC
with
the XNewQualifiedOperators
option. In the new syntax, the prefix form of a qualified
operator is
written
(in Haskell 98 this would
be module
.(symbol
)(
),
and the infix form is
written module
.symbol
)`
(in Haskell 98 this would
be module
.(symbol
)``
.
For example:
module
.symbol
`
add x y = Prelude.(+) x y subtract y = (`Prelude.()` y)
The new form of qualified operators is intended to regularise
the syntax by eliminating odd cases
like Prelude..
. For example,
when NewQualifiedOperators
is on, it is possible to
write the enumerated sequence [Monday..]
without spaces, whereas in Haskell 98 this would be a
reference to the operator ‘.
‘
from module Monday
.
When XNewQualifiedOperators
is on, the old Haskell
98 syntax for qualified operators is not accepted, so this
option may cause existing Haskell 98 code to break.
GHC supports a small extension to the syntax of module
names: a module name is allowed to contain a dot
‘.’
. This is also known as the
“hierarchical module namespace” extension, because
it extends the normally flat Haskell module namespace into a
more flexible hierarchy of modules.
This extension has very little impact on the language
itself; modules names are always fully
qualified, so you can just think of the fully qualified module
name as “the module name”. In particular, this
means that the full module name must be given after the
module
keyword at the beginning of the
module; for example, the module A.B.C
must
begin
module A.B.C
It is a common strategy to use the as
keyword to save some typing when using qualified names with
hierarchical modules. For example:
import qualified Control.Monad.ST.Strict as ST
For details on how GHC searches for source and interface files in the presence of hierarchical modules, see Section 4.6.3, “The search path”.
GHC comes with a large collection of libraries arranged hierarchically; see the accompanying library documentation. More libraries to install are available from HackageDB.
The discussion that follows is an abbreviated version of Simon Peyton Jones's original proposal. (Note that the proposal was written before pattern guards were implemented, so refers to them as unimplemented.)
Suppose we have an abstract data type of finite maps, with a lookup operation:
lookup :: FiniteMap > Int > Maybe Int
The lookup returns Nothing
if the supplied key is not in the domain of the mapping, and (Just v)
otherwise,
where v
is the value that the key maps to. Now consider the following definition:
clunky env var1 var2  ok1 && ok2 = val1 + val2  otherwise = var1 + var2 where m1 = lookup env var1 m2 = lookup env var2 ok1 = maybeToBool m1 ok2 = maybeToBool m2 val1 = expectJust m1 val2 = expectJust m2
The auxiliary functions are
maybeToBool :: Maybe a > Bool maybeToBool (Just x) = True maybeToBool Nothing = False expectJust :: Maybe a > a expectJust (Just x) = x expectJust Nothing = error "Unexpected Nothing"
What is clunky
doing? The guard ok1 &&
ok2
checks that both lookups succeed, using
maybeToBool
to convert the Maybe
types to booleans. The (lazily evaluated) expectJust
calls extract the values from the results of the lookups, and binds the
returned values to val1
and val2
respectively. If either lookup fails, then clunky takes the
otherwise
case and returns the sum of its arguments.
This is certainly legal Haskell, but it is a tremendously verbose and unobvious way to achieve the desired effect. Arguably, a more direct way to write clunky would be to use case expressions:
clunky env var1 var2 = case lookup env var1 of Nothing > fail Just val1 > case lookup env var2 of Nothing > fail Just val2 > val1 + val2 where fail = var1 + var2
This is a bit shorter, but hardly better. Of course, we can rewrite any set
of patternmatching, guarded equations as case expressions; that is
precisely what the compiler does when compiling equations! The reason that
Haskell provides guarded equations is because they allow us to write down
the cases we want to consider, one at a time, independently of each other.
This structure is hidden in the case version. Two of the righthand sides
are really the same (fail
), and the whole expression
tends to become more and more indented.
Here is how I would write clunky:
clunky env var1 var2  Just val1 < lookup env var1 , Just val2 < lookup env var2 = val1 + val2 ...other equations for clunky...
The semantics should be clear enough. The qualifiers are matched in order.
For a <
qualifier, which I call a pattern guard, the
right hand side is evaluated and matched against the pattern on the left.
If the match fails then the whole guard fails and the next equation is
tried. If it succeeds, then the appropriate binding takes place, and the
next qualifier is matched, in the augmented environment. Unlike list
comprehensions, however, the type of the expression to the right of the
<
is the same as the type of the pattern to its
left. The bindings introduced by pattern guards scope over all the
remaining guard qualifiers, and over the right hand side of the equation.
Just as with list comprehensions, boolean expressions can be freely mixed with among the pattern guards. For example:
f x  [y] < x , y > 3 , Just z < h y = ...
Haskell's current guards therefore emerge as a special case, in which the qualifier list has just one element, a boolean expression.
View patterns are enabled by the flag XViewPatterns
.
More information and examples of view patterns can be found on the
Wiki
page.
View patterns are somewhat like pattern guards that can be nested inside of other patterns. They are a convenient way of patternmatching 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:
type Typ data TypView = Unit  Arrow Typ Typ view :: Type > TypeView  additional operations for constructing Typ's ...
The representation of Typ is held abstract, permitting implementations to use a fancy representation (e.g., hashconsing to manage sharing). Without view patterns, using this signature a little inconvenient:
size :: Typ > Integer size t = case view t of Unit > 1 Arrow t1 t2 > size t1 + size t2
It is necessary to iterate the case, rather than using an equational
function definition. And the situation is even worse when the matching
against t
is buried deep inside another pattern.
View patterns permit calling the view function inside the pattern and matching against the result:
size (view > Unit) = 1 size (view > Arrow t1 t2) = size t1 + size t2
That is, we add a new form of pattern, written
expression
>
pattern
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.
The semantics of a pattern (
exp
>
pat
)
are as follows:
The variables bound by the view pattern are the variables bound by
pat
.
Any variables in exp
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
clunky
from Section 7.3.4, “Pattern guards” can be
written using view patterns as follows:
clunky env (lookup env > Just val1) (lookup env > Just val2) = val1 + val2 ...other equations for clunky...
More precisely, the scoping rules are:
In a single pattern, variables bound by patterns to the left of a view pattern expression are in scope. For example:
example :: Maybe ((String > Integer,Integer), String) > Bool example Just ((f,_), f > 4) = True
Additionally, in function definitions, variables bound by matching earlier curried arguments may be used in view pattern expressions in later arguments:
example :: (String > Integer) > String > Bool example f (f > 4) = True
That is, the scoping is the same as it would be if the curried arguments were collected into a tuple.
In mutually recursive bindings, such as let
,
where
, or the top level, view patterns in one
declaration may not mention variables bound by other declarations. That
is, each declaration must be selfcontained. For example, the following
program is not allowed:
let {(x > y) = e1 ; (y > x) = e2 } in x
(We may lift this restriction in the future; the only cost is that type checking patterns would get a little more complicated.)
Typing: If exp
has type
T1
>
T2
and pat
matches
a T2
, then the whole view pattern matches a
T1
.
Matching: To the equations in Section 3.17.3 of the Haskell 98 Report, add the following:
case v of { (e > p) > e1 ; _ > e2 } = case (e v) of { p > e1 ; _ > e2 }
That is, to match a variable v
against a pattern
(
exp
>
pat
)
, evaluate (
exp
v
)
and match the result against
pat
.
Efficiency: When the same view function is applied in
multiple branches of a function definition or a case expression (e.g.,
in size
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 The
Implementation of Functional Programming Languages. 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
f ((view > A, p1), p2) = e1 f ((view > B, p3), p4) = e2
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 view ("hi", "there")
will be
collected. However, the current implementation does not compare up to
alphaequivalence, so two instances of (x, view x >
y)
will not be coalesced.
The recursive donotation (also known as mdonotation) is implemented as described in A recursive do for Haskell, by Levent Erkok, John Launchbury, Haskell Workshop 2002, pages: 2937. Pittsburgh, Pennsylvania. This paper is essential reading for anyone making nontrivial use of mdonotation, and we do not repeat it here.
The donotation of Haskell does not allow recursive bindings, that is, the variables bound in a doexpression are visible only in the textually following code block. Compare this to a letexpression, where bound variables are visible in the entire binding group. It turns out that several applications can benefit from recursive bindings in the donotation, and this extension provides the necessary syntactic support.
Here is a simple (yet contrived) example:
import Control.Monad.Fix justOnes = mdo xs < Just (1:xs) return xs
As you can guess justOnes
will evaluate to Just [1,1,1,...
.
The Control.Monad.Fix library introduces the MonadFix
class. It's definition is:
class Monad m => MonadFix m where mfix :: (a > m a) > m a
The function mfix
dictates how the required recursion operation should be performed. For example,
justOnes
desugars as follows:
justOnes = mfix (\xs' > do { xs < Just (1:xs'); return xs }
For full details of the way in which mdo is typechecked and desugared, see the paper A recursive do for Haskell. In particular, GHC implements the segmentation technique described in Section 3.2 of the paper.
If recursive bindings are required for a monad,
then that monad must be declared an instance of the MonadFix
class.
The following instances of MonadFix
are automatically provided: List, Maybe, IO.
Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class
for Haskell's internal state monad (strict and lazy, respectively).
Here are some important points in using the recursivedo notation:
The recursive version of the donotation uses the keyword mdo
(rather
than do
).
It is enabled with the flag XRecursiveDo
, which is in turn implied by
fglasgowexts
.
Unlike ordinary donotation, but like let
and where
bindings,
name shadowing is not allowed; that is, all the names bound in a single mdo
must
be distinct (Section 3.3 of the paper).
Variables bound by a let
statement in an mdo
are monomorphic in the mdo
(Section 3.1 of the paper). However
GHC breaks the mdo
into segments to enhance polymorphism,
and improve termination (Section 3.2 of the paper).
Historical note: The old implementation of the mdonotation (and most
of the existing documents) used the name
MonadRec
for the class and the corresponding library.
This name is not supported by GHC.
Parallel list comprehensions are a natural extension to list comprehensions. List comprehensions can be thought of as a nice syntax for writing maps and filters. Parallel comprehensions extend this to include the zipWith family.
A parallel list comprehension has multiple independent branches of qualifier lists, each separated by a `' symbol. For example, the following zips together two lists:
[ (x, y)  x < xs  y < ys ]
The behavior of parallel list comprehensions follows that of zip, in that the resulting list will have the same length as the shortest branch.
We can define parallel list comprehensions by translation to regular comprehensions. Here's the basic idea:
Given a parallel comprehension of the form:
[ e  p1 < e11, p2 < e12, ...  q1 < e21, q2 < e22, ... ... ]
This will be translated to:
[ e  ((p1,p2), (q1,q2), ...) < zipN [(p1,p2)  p1 < e11, p2 < e12, ...] [(q1,q2)  q1 < e21, q2 < e22, ...] ... ]
where `zipN' is the appropriate zip for the given number of branches.
Generalised list comprehensions are a further enhancement to the list comprehension syntatic sugar to allow operations such as sorting and grouping which are familiar from SQL. They are fully described in the paper Comprehensive comprehensions: comprehensions with "order by" and "group by", except that the syntax we use differs slightly from the paper.
Here is an example:
employees = [ ("Simon", "MS", 80) , ("Erik", "MS", 100) , ("Phil", "Ed", 40) , ("Gordon", "Ed", 45) , ("Paul", "Yale", 60)] output = [ (the dept, sum salary)  (name, dept, salary) < employees , then group by dept , then sortWith by (sum salary) , then take 5 ]
In this example, the list output
would take on
the value:
[("Yale", 60), ("Ed", 85), ("MS", 180)]
There are three new keywords: group
, by
, and using
.
(The function sortWith
is not a keyword; it is an ordinary
function that is exported by GHC.Exts
.)
There are five new forms of comprehension qualifier,
all introduced by the (existing) keyword then
:
then fThis statement requires that
f
have the type
forall a. [a] > [a]
. You can see an example of it's use in the
motivating example, as this form is used to apply take 5
.
then f by e
This form is similar to the previous one, but allows you to create a function
which will be passed as the first argument to f. As a consequence f must have
the type forall a. (a > t) > [a] > [a]
. As you can see
from the type, this function lets f "project out" some information
from the elements of the list it is transforming.
An example is shown in the opening example, where sortWith
is supplied with a function that lets it find out the sum salary
for any item in the list comprehension it transforms.
then group by e using f
This is the most general of the groupingtype statements. In this form,
f is required to have type forall a. (a > t) > [a] > [[a]]
.
As with the then f by e
case above, the first argument
is a function supplied to f by the compiler which lets it compute e on every
element of the list being transformed. However, unlike the nongrouping case,
f additionally partitions the list into a number of sublists: this means that
at every point after this statement, binders occurring before it in the comprehension
refer to lists of possible values, not single values. To help understand
this, let's look at an example:
 This works similarly to groupWith in GHC.Exts, but doesn't sort its input first groupRuns :: Eq b => (a > b) > [a] > [[a]] groupRuns f = groupBy (\x y > f x == f y) output = [ (the x, y)  x < ([1..3] ++ [1..2]) , y < [4..6] , then group by x using groupRuns ]
This results in the variable output
taking on the value below:
[(1, [4, 5, 6]), (2, [4, 5, 6]), (3, [4, 5, 6]), (1, [4, 5, 6]), (2, [4, 5, 6])]
Note that we have used the the
function to change the type
of x from a list to its original numeric type. The variable y, in contrast, is left
unchanged from the list form introduced by the grouping.
then group by e
This form of grouping is essentially the same as the one described above. However,
since no function to use for the grouping has been supplied it will fall back on the
groupWith
function defined in
GHC.Exts
. This
is the form of the group statement that we made use of in the opening example.
then group using f
With this form of the group statement, f is required to simply have the type
forall a. [a] > [[a]]
, which will be used to group up the
comprehension so far directly. An example of this form is as follows:
output = [ x  y < [1..5] , x < "hello" , then group using inits]
This will yield a list containing every prefix of the word "hello" written out 5 times:
["","h","he","hel","hell","hello","helloh","hellohe","hellohel","hellohell","hellohello","hellohelloh",...]
GHC normally imports
Prelude.hi
files for you. If you'd
rather it didn't, then give it a
XNoImplicitPrelude
option. The idea is
that you can then import a Prelude of your own. (But don't
call it Prelude
; the Haskell module
namespace is flat, and you must not conflict with any
Prelude module.)
Suppose you are importing a Prelude of your own
in order to define your own numeric class
hierarchy. It completely defeats that purpose if the
literal "1" means "Prelude.fromInteger
1
", which is what the Haskell Report specifies.
So the XNoImplicitPrelude
flag also causes
the following pieces of builtin syntax to refer to
whatever is in scope, not the Prelude
versions:
An integer literal 368
means
"fromInteger (368::Integer)
", rather than
"Prelude.fromInteger (368::Integer)
".
Fractional literals are handed in just the same way,
except that the translation is
fromRational (3.68::Rational)
.
The equality test in an overloaded numeric pattern
uses whatever (==)
is in scope.
The subtraction operation, and the
greaterthanorequal test, in n+k
patterns
use whatever ()
and (>=)
are in scope.
Negation (e.g. " (f x)
")
means "negate (f x)
", both in numeric
patterns, and expressions.
"Do" notation is translated using whatever
functions (>>=)
,
(>>)
, and fail
,
are in scope (not the Prelude
versions). List comprehensions, mdo (Section 7.3.6, “The recursive donotation
”), and parallel array
comprehensions, are unaffected.
Arrow
notation (see Section 7.10, “Arrow notation
”)
uses whatever arr
,
(>>>)
, first
,
app
, ()
and
loop
functions are in scope. But unlike the
other constructs, the types of these functions must match the
Prelude types very closely. Details are in flux; if you want
to use this, ask!
In all cases (apart from arrow notation), the static semantics should be that of the desugared form,
even if that is a little unexpected. For example, the
static semantics of the literal 368
is exactly that of fromInteger (368::Integer)
; it's fine for
fromInteger
to have any of the types:
fromInteger :: Integer > Integer fromInteger :: forall a. Foo a => Integer > a fromInteger :: Num a => a > Integer fromInteger :: Integer > Bool > Bool
Be warned: this is an experimental facility, with
fewer checks than usual. Use dcorelint
to typecheck the desugared program. If Core Lint is happy
you should be all right.
The XPostfixOperators
flag enables 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.
The extension does not extend to the lefthand side of function definitions; you must define such a function in prefix form.
In record construction and record pattern matching it is entirely unambiguous which field is referred to, even if there are two different data types in scope with a common field name. For example:
module M where data S = MkS { x :: Int, y :: Bool } module Foo where import M data T = MkT { x :: Int } ok1 (MkS { x = n }) = n+1  Unambiguous ok2 n = MkT { x = n+1 }  Unambiguous bad1 k = k { x = 3 }  Ambiguous bad2 k = x k  Ambiguous
Even though there are two x
's in scope,
it is clear that the x
in the pattern in the
definition of ok1
can only mean the field
x
from type S
. Similarly for
the function ok2
. However, in the record update
in bad1
and the record selection in bad2
it is not clear which of the two types is intended.
Haskell 98 regards all four as ambiguous, but with the
XDisambiguateRecordFields
flag, GHC will accept
the former two. The rules are precisely the same as those for instance
declarations in Haskell 98, where the method names on the lefthand side
of the method bindings in an instance declaration refer unambiguously
to the method of that class (provided they are in scope at all), even
if there are other variables in scope with the same name.
This reduces the clutter of qualified names when you import two
records from different modules that use the same field name.
Record puns are enabled by the flag XNamedFieldPuns
.
When using records, it is common to write a pattern that binds a variable with the same name as a record field, such as:
data C = C {a :: Int} f (C {a = a}) = a
Record punning permits the variable name to be elided, so one can simply write
f (C {a}) = a
to mean the same pattern as above. That is, in a record pattern, the
pattern a
expands into the pattern a =
a
for the same name a
.
Note that puns and other patterns can be mixed in the same record:
data C = C {a :: Int, b :: Int} f (C {a, b = 4}) = a
and that puns can be used wherever record patterns occur (e.g. in
let
bindings or at the toplevel).
Record punning can also be used in an expression, writing, for example,
let a = 1 in C {a}
instead of
let a = 1 in C {a = a}
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.
Record wildcards are enabled by the flag XRecordWildCards
.
For records with many fields, it can be tiresome to write out each field individually in a record pattern, as in
data C = C {a :: Int, b :: Int, c :: Int, d :: Int} f (C {a = 1, b = b, c = c, d = d}) = b + c + d
Record wildcard syntax permits a (..
) in a record
pattern, where each elided field f
is replaced by the
pattern f = f
. For example, the above pattern can be
written as
f (C {a = 1, ..}) = b + c + d
Note that wildcards can be mixed with other patterns, including puns
(Section 7.3.12, “Record puns
”); for example, in a pattern C {a
= 1, b, ..})
. Additionally, record wildcards can be used
wherever record patterns occur, including in let
bindings and at the toplevel. For example, the toplevel binding
C {a = 1, ..} = e
defines b
, c
, and
d
.
Record wildcards can also be used in expressions, writing, for example,
let {a = 1; b = 2; c = 3; d = 4} in C {..}
in place of
let {a = 1; b = 2; c = 3; d = 4} in C {a=a, b=b, c=c, d=d}
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.
A careful reading of the Haskell 98 Report reveals that fixity
declarations (infix
, infixl
, and
infixr
) are permitted to appear inside local bindings
such those introduced by let
and
where
. However, the Haskell Report does not specify
the semantics of such bindings very precisely.
In GHC, a fixity declaration may accompany a local binding:
let f = ... infixr 3 `f` in ...
and the fixity declaration applies wherever the binding is in scope.
For example, in a let
, it applies in the righthand
sides of other let
bindings and the body of the
let
C. Or, in recursive do
expressions (Section 7.3.6, “The recursive donotation
”), the local fixity
declarations of a let
statement scope over other
statements in the group, just as the bound name does.
Moreover, a local fixity declaration *must* accompany a local binding of that name: it is not possible to revise the fixity of name bound elsewhere, as in
let infixr 9 $ in ...
Because local fixity declarations are technically Haskell 98, no flag is necessary to enable them.
With the XPackageImports
flag, GHC allows
import declarations to be qualified by the package name that the
module is intended to be imported from. For example:
import "network" Network.Socket
would import the module Network.Socket
from
the package network
(any version). This may
be used to disambiguate an import when the same module is
available from multiple packages, or is present in both the
current package being built and an external package.
Note: you probably don't need to use this feature, it was added mainly so that we can build backwardscompatible versions of packages when APIs change. It can lead to fragile dependencies in the common case: modules occasionally move from one package to another, rendering any packagequalified imports broken.
Turning on an option that enables special syntax might cause working Haskell 98 code to fail to compile, perhaps because it uses a variable name which has become a reserved word. This section lists the syntax that is "stolen" by language extensions. We use notation and nonterminal names from the Haskell 98 lexical syntax (see the Haskell 98 Report). We only list syntax changes here that might affect existing working programs (i.e. "stolen" syntax). Many of these extensions will also enable new contextfree syntax, but in all cases programs written to use the new syntax would not be compilable without the option enabled.
There are two classes of special syntax:
New reserved words and symbols: character sequences which are no longer available for use as identifiers in the program.
Other special syntax: sequences of characters that have a different meaning when this particular option is turned on.
The following syntax is stolen:
forall
Stolen (in types) by: XScopedTypeVariables
,
XLiberalTypeSynonyms
,
XRank2Types
,
XRankNTypes
,
XPolymorphicComponents
,
XExistentialQuantification
mdo
Stolen by: XRecursiveDo
,
foreign
Stolen by: XForeignFunctionInterface
,
rec
,
proc
, <
,
>
, <<
,
>>
, and (
,
)
brackets
Stolen by: XArrows
,
?varid
,
%varid
Stolen by: XImplicitParams
,
[
,
[e
, [p
,
[d
, [t
,
$(
,
$varid
Stolen by: XTemplateHaskell
,
[:varid

Stolen by: XQuasiQuotes
,
varid
{#
},
char
#
,
string
#
,
integer
#
,
float
#
,
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Stolen by: XMagicHash
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