6.14. Bang patterns and Strict Haskell

In high-performance 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 (call-by-value) evaluation rather than lazy (call-by-need) evaluation.

  • Bang patterns (BangPatterns) makes pattern matching and let bindings stricter.
  • Strict data types (StrictData) makes constructor fields strict by default, on a per-module basis.
  • Strict pattern (Strict) makes all patterns and let bindings strict by default, on a per-module basis.

The latter two extensions are simply a way to avoid littering high-performance 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

Status:Included in GHC2021

Allow use of bang pattern syntax.

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 in x but not in y.

  • 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 and g2 mean exactly the same thing. But g3 evaluates (f x), binds y to the result, and then evaluates body.

  • Bang patterns do not have any effect with constructor patterns:

    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. 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. 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) = ... and f4 !(x,y) = .... are equivalent (because the constructor pattern (x,y) forces the argument), but the expressions let (p,q) = e in body and let !(p,q) = e in body are different. The former will not evaluate e unless p or q is forced in body.

  • Only a top-level 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 or y is evaluated by b the entire pattern is matched, including forcing the evaluation of x.

  • Because the ! in a strict binding is not a bang pattern, it must be visible without looking through pattern synonyms

    pattern 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. Strict-by-default data types


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. Strict-by-default pattern bindings


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 of x gives the regular lazy behavior.

    Turning patterns into irrefutable ones requires ~(~p) when Strict 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 of x 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, do-notation, etc

    The outermost pattern of all pattern matches gets an implicit bang, unless disabled with ~. This applies to case expressions, patterns in lambda, do-notation, list comprehension, and so on. For example

    case 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 as

    case x of !y -> rhs

    which evaluates x. Similarly, if newtype Age = MkAge Int, then

    case x of MkAge i -> rhs

    is lazy in Haskell; but with Strict the added bang makes it strict.


    \ 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 and q to the components of the pair. But the pair itself is lazy (unless we also compile the Prelude with Strict; see Modularity below). So p and q 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 in x; and g is strict in its argument and hence also strict in x. With Strict, both become strict because f’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 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. Replace the “Translation” there with the following one. Given let { bind1 ... bindn } in body:


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.


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 non-recursive 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 non-recursive 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 non-recursive case:

let x :: Int     -- Non-recursive
    !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