Copyright | Nils Anders Danielsson 2006 Alexander Berntsen 2014 |
---|---|
License | BSD-style (see the LICENSE file in the distribution) |
Maintainer | libraries@haskell.org |
Stability | experimental |
Portability | portable |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
- Prelude re-exports
- Other combinators
Simple combinators working solely on and with functions.
Prelude re-exports
const x
is a unary function which evaluates to x
for all inputs.
>>>
const 42 "hello"
42
>>>
map (const 42) [0..3]
[42,42,42,42]
flip :: (a -> b -> c) -> b -> a -> c Source #
takes its (first) two arguments in the reverse order of flip
ff
.
>>>
flip (++) "hello" "world"
"worldhello"
($) :: forall r a (b :: TYPE r). (a -> b) -> a -> b infixr 0 Source #
Application operator. This operator is redundant, since ordinary
application (f x)
means the same as (f
. However, $
x)$
has
low, right-associative binding precedence, so it sometimes allows
parentheses to be omitted; for example:
f $ g $ h x = f (g (h x))
It is also useful in higher-order situations, such as
,
or map
($
0) xs
.zipWith
($
) fs xs
Note that ($)
is levity-polymorphic in its result type, so that
foo $ True where foo :: Bool -> Int#
is well-typed
Other combinators
is the least fixed point of the function fix
ff
,
i.e. the least defined x
such that f x = x
.
For example, we can write the factorial function using direct recursion as
>>>
let fac n = if n <= 1 then 1 else n * fac (n-1) in fac 5
120
This uses the fact that Haskell’s let
introduces recursive bindings. We can
rewrite this definition using fix
,
>>>
fix (\rec n -> if n <= 1 then 1 else n * rec (n-1)) 5
120
Instead of making a recursive call, we introduce a dummy parameter rec
;
when used within fix
, this parameter then refers to fix'
argument, hence
the recursion is reintroduced.
on :: (b -> b -> c) -> (a -> b) -> a -> a -> c infixl 0 Source #