GHC supports an extension of pattern matching called *bang
patterns*, written `!`

.
Bang patterns are under consideration for Haskell Prime.
The Haskell
prime feature description contains more discussion and examples
than the material below.
`pat`

The key change is the addition of a new rule to the
semantics of pattern matching in the Haskell 98 report.
Add new bullet 10, saying: Matching the pattern `!`

* pat*
against a value

`v`

if

is bottom, the match diverges`v`

otherwise,

is matched against`pat`

`v`

Bang patterns are enabled by the flag `-XBangPatterns`

.

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.

There is one (apparent) exception to this general rule that a bang only
makes a difference when it precedes a variable or wild-card: a bang at the
top level of a `let`

or `where`

binding makes the binding strict, regardless of the pattern. For example:

let ![x,y] = e in b

is a strict binding: operationally, it evaluates `e`

, matches
it against the pattern `[x,y]`

, and then evaluates `b`

.
(We say "apparent" exception because the Right Way to think of it is that the bang
at the top of a binding is not part of the *pattern*; rather it
is part of the syntax of the *binding*.)
Nested bangs in a pattern binding behave uniformly with all other forms of
pattern matching. For example

let (!x,[y]) = e in b

is equivalent to this:

let { t = case e of (x,[y]) -> x `seq` (x,y) x = fst t y = snd t } in b

The binding is lazy, but when either `x`

or `y`

is
evaluated by `b`

the entire pattern is matched, including forcing the
evaluation of `x`

.

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`

evaluates `(f x)`

, binds `y`

to the
result, and then evaluates `body`

.

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 in 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 divergesotherwise,

`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.