6.14. Bang patterns and Strict Haskell¶
In highperformance Haskell code (e.g. numeric code) eliminating thunks from an inner loop can be a huge win. GHC supports three extensions to allow the programmer to specify use of strict (callbyvalue) evaluation rather than lazy (callbyneed) evaluation.
 Bang patterns (
BangPatterns
) makes pattern matching and let bindings stricter.  Strict data types (
StrictData
) makes constructor fields strict by default, on a permodule basis.  Strict pattern (
Strict
) makes all patterns and let bindings strict by default, on a permodule basis.
The latter two extensions are simply a way to avoid littering highperformance code with bang patterns, making it harder to read.
Bang patterns and strict matching do not affect the type system in any way.
6.14.1. Bang patterns¶
GHC supports an extension of pattern matching called bang patterns,
written !pat
. Bang patterns are available by default as a part
of GHC2021
.
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.
Note the following points:
Bang patterns can be nested:
f2 (!x, y) = [x,y]
Here,
f2
is strict inx
but not iny
.Bang patterns can be used in
case
expressions too:g1 x = let y = f x in body g2 x = case f x of { y > body } g3 x = case f x of { !y > body }
The functions
g1
andg2
mean exactly the same thing. Butg3
evaluates(f x)
, bindsy
to the result, and then evaluatesbody
.Bang patterns do not have any effect with constructor patterns:
f3 !(x,y) = [x,y] f4 (x,y) = [x,y]
Here,
f3
andf4
are identical; putting a bang before a pattern that forces evaluation anyway does nothing. However, see the caveat below.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 by looking at the surrounding whitespace:a ! b = ...  infix operator a !b = ...  bang pattern
See GHC Proposal #229 for the precise rules.
6.14.1.1. Strict bindings¶
The BangPatterns
extension furthermore enables syntax for strict
let
or where
bindings with !pat = expr
. For example,
let !x = e in body
let !(p,q) = e in body
In both cases e
is evaluated before starting to evaluate body
.
Note the following points:
This form is not the same as a bang pattern: The declarations
f3 (x,y) = ...
andf4 !(x,y) = ....
are equivalent (because the constructor pattern(x,y)
forces the argument), but the expressionslet (p,q) = e in body
andlet !(p,q) = e in body
are different. The former will not evaluatee
unlessp
orq
is forced inbody
.Only a toplevel bang (perhaps under parentheses) makes the binding strict; otherwise, it is considered a normal bang pattern. 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
ory
is evaluated byb
the entire pattern is matched, including forcing the evaluation ofx
.Because the
!
in a strict binding is not a bang pattern, it must be visible without looking through pattern synonymspattern Bang x < !x f1 = let Bang x = y in ... f2 = let !x = y in ...  not equivalent to f1
Strict bindings are not allowed at the top level of a module.
See Semantics of let bindings with bang patterns for the detailed semantics, and the Haskell prime feature description for more discussion and examples.
6.14.2. Strictbydefault data types¶

StrictData
¶ Since: 8.0.1 Make fields of data types defined in the current module strict by default.
Informally the StrictData
language extension switches data type
declarations to be strict by default allowing fields to be lazy by
adding a ~
in front of the field.
When the user writes
data T = C a
data T' = C' ~a
we interpret it as if they had written
data T = C !a
data T' = C' a
The extension only affects definitions in this module.
The ~
annotation must be written in prefix form:
data T = MkT ~Int  valid
data T = MkT ~ Int  invalid
See GHC Proposal #229 for the precise rules.
6.14.3. Strictbydefault pattern bindings¶

Strict
¶ Implies: StrictData
Since: 8.0.1 Make bindings in the current module strict by default.
Informally the Strict
language extension switches functions, data
types, and bindings to be strict by default, allowing optional laziness
by adding ~
in front of a variable. This essentially reverses the
present situation where laziness is default and strictness can be
optionally had by adding !
in front of a variable.
Strict
implies StrictData.
Function definitions
When the user writes
f x = ...
we interpret it as if they had written
f !x = ...
Adding
~
in front ofx
gives the regular lazy behavior.Turning patterns into irrefutable ones requires
~(~p)
whenStrict
is enabled.Let/where bindings
When the user writes
let x = ... let pat = ...
we interpret it as if they had written
let !x = ... let !pat = ...
Adding
~
in front ofx
gives the regular lazy behavior. The general rule is that we add an implicit bang on the outermost pattern, unless disabled with~
.Pattern matching in case expressions, lambdas, donotation, etc
The outermost pattern of all pattern matches gets an implicit bang, unless disabled with
~
. This applies to case expressions, patterns in lambda, donotation, list comprehension, and so on. For examplecase x of (a,b) > rhs
is interpreted as
case x of !(a,b) > rhs
Since the semantics of pattern matching in case expressions is strict, this usually has no effect whatsoever. But it does make a difference in the degenerate case of variables and newtypes. So
case x of y > rhs
is lazy in Haskell, but with
Strict
is interpreted ascase x of !y > rhs
which evaluates
x
. Similarly, ifnewtype Age = MkAge Int
, thencase x of MkAge i > rhs
is lazy in Haskell; but with
Strict
the added bang makes it strict.Similarly
\ x > body do { x < rhs; blah } [ e  x < rhs; blah }
all get implicit bangs on the
x
pattern.Nested patterns
Notice that we do not put bangs on nested patterns. For example
let (p,q) = if flob then (undefined, undefined) else (True, False) in ...
will behave like
let !(p,q) = if flob then (undefined, undefined) else (True,False) in ...
which will strictly evaluate the right hand side, and bind
p
andq
to the components of the pair. But the pair itself is lazy (unless we also compile thePrelude
withStrict
; see Modularity below). Sop
andq
may end up bound to undefined. See also Dynamic semantics of bang patterns below.Top level bindings
are unaffected by
Strict
. For example:x = factorial 20 (y,z) = if x > 10 then True else False
Here
x
and the pattern binding(y,z)
remain lazy. Reason: there is no good moment to force them, until first use.Newtypes
There is no effect on newtypes, which simply rename existing types. For example:
newtype T = C a f (C x) = rhs1 g !(C x) = rhs2
In ordinary Haskell,
f
is lazy in its argument and hence inx
; andg
is strict in its argument and hence also strict inx
. WithStrict
, both become strict becausef
’s argument gets an implicit bang.
6.14.4. Modularity¶
Strict
and StrictData
only affects definitions in the module
they are used in. Functions and data types imported from other modules
are unaffected. For example, we won’t evaluate the argument to
Just
before applying the constructor. Similarly we won’t evaluate
the first argument to Data.Map.findWithDefault
before applying the
function.
This is crucial to preserve correctness. Entities defined in other modules might rely on laziness for correctness (whether functional or performance).
Tuples, lists, Maybe
, and all the other types from Prelude
continue to have their existing, lazy, semantics.
6.14.5. Dynamic semantics of bang patterns¶
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 valuev
behaves as follows: if
v
is bottom, the match diverges  otherwise,
pat
is matched againstv
 if
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.
Replace the “Translation” there with the following one. Given
let { bind1 ... bindn } in body
:
FORCE
Replace any binding !p = e
with v = case e of p > (x1, ..., xn); (x1, ..., xn) = v
and replace
body
with v seq body
, where v
is fresh. This translation works fine if
p
is already a variable x
, but can obviously be optimised by not
introducing a fresh variable v
.
SPLIT
Replace any binding p = e
, where p
is not a variable, with
v = e; x1 = case v of p > x1; ...; xn = case v of p > xn
, where
v
is fresh and x1
.. xn
are the bound variables of p
.
Again if e
is a variable, this can be optimised by not introducing a
fresh variable.
The result will be a (possibly) recursive set of bindings, binding
only simple variables on the left hand side. (One could go one step
further, as in the Haskell Report and make the recursive bindings
nonrecursive using fix
, but we do not do so in Core, and it only
obfuscates matters, so we do not do so here.)
The translation is carefully crafted to make bang patterns meaningful for recursive and polymorphic bindings as well as straightforward nonrecursive bindings.
Here are some examples of how this translation works. The first expression of each sequence is Haskell source; the subsequent ones are Core.
Here is a simple nonrecursive case:
let x :: Int  Nonrecursive
!x = factorial y
in body
===> (FORCE)
let x = factorial y in x `seq` body
===> (inline seq)
let x = factorial y in case x of x > body
===> (inline x)
case factorial y of x > body
Same again, only with a pattern binding:
let !(Just x, Left y) = e in body
===> (FORCE)
let v = case e of (Just x, Left y) > (x,y)
(x,y) = v
in v `seq` body
===> (SPLIT)
let v = case e of (Just x, Left y) > (x,y)
x = case v of (x,y) > x
y = case v of (x,y) > y
in v `seq` body
===> (inline seq, float x,y bindings inwards)
let v = case e of (Just x, Left y) > (x,y)
in case v of v > let x = case v of (x,y) > x
y = case v of (x,y) > y
in body
===> (fluff up v's pattern; this is a standard Core optimisation)
let v = case e of (Just x, Left y) > (x,y)
in case v of v@(p,q) > let x = case v of (x,y) > x
y = case v of (x,y) > y
in body
===> (case of known constructor)
let v = case e of (Just x, Left y) > (x,y)
in case v of v@(p,q) > let x = p
y = q
in body
===> (inline x,y, v)
case (case e of (Just x, Left y) > (x,y) of
(p,q) > body[p/x, q/y]
===> (case of case)
case e of (Just x, Left y) > body[p/x, q/y]
The final form is just what we want: a simple case expression.
Here is a recursive case
letrec xs :: [Int]  Recursive
!xs = factorial y : xs
in body
===> (FORCE)
letrec xs = factorial y : xs in xs `seq` body
===> (inline seq)
letrec xs = factorial y : xs in case xs of xs > body
===> (eliminate case of value)
letrec xs = factorial y : xs in body
and a polymorphic one:
let f :: forall a. [a] > [a]  Polymorphic
!f = fst (reverse, True)
in body
===> (FORCE)
let f = /\a. fst (reverse a, True) in f `seq` body
===> (inline seq, inline f)
case (/\a. fst (reverse a, True)) of f > body
Notice that the seq
is added only in the translation to Core
If we did it in Haskell source, thus
let f = ... in f `seq` body
then f
’s polymorphic type would get instantiated, so the Core
translation would be
let f = ... in f Any `seq` body
When overloading is involved, the results might be slightly counter intuitive:
let f :: forall a. Eq a => a > [a] > Bool  Overloaded
!f = fst (member, True)
in body
===> (FORCE)
let f = /\a \(d::Eq a). fst (member, True) in f `seq` body
===> (inline seq, case of value)
let f = /\a \(d::Eq a). fst (member, True) in body
Note that the bang has no effect at all in this case