Applying one function to the result of another is known as function composition: `(f.g)(x) = f(g(x))`
Applying one function to the result of another is known as function composition: (f.g)(x) = f(g(x)).
See also pointfree.
Questions tagged [function-composition]
370 questions
747
votes
13 answers
What is the difference between . (dot) and $ (dollar sign)?
What is the difference between the dot (.) and the dollar sign ($)?
As I understand it, they are both syntactic sugar for not needing to use parentheses.
Rabarberski
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133
votes
7 answers
Haskell function composition (.) and function application ($) idioms: correct use
I have been reading Real World Haskell, and I am nearing the end, but a matter of style has been niggling at me to do with the (.) and ($) operators.
When you write a function that is a composition of other functions you write it like:
f = g .…
Robert Massaioli
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90
votes
6 answers
Dot Operator in Haskell: need more explanation
I'm trying to understand what the dot operator is doing in this Haskell code:
sumEuler = sum . (map euler) . mkList
The entire source code is below.
My understanding
The dot operator is taking the two functions sum and the result of map euler and…
cbrulak
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54
votes
3 answers
runST and function composition
Why does this typecheck:
runST $ return $ True
While the following does not:
runST . return $ True
GHCI complains:
Couldn't match expected type `forall s. ST s c0'
with actual type `m0 a0'
Expected type: a0 -> forall s. ST s c0
…
Grzegorz Chrupała
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49
votes
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Haskell function composition operator of type (c→d) → (a→b→c) → (a→b→d)
Ordinary function composition is of the type
(.) :: (b -> c) -> (a -> b) -> a -> c
I figure this should generalize to types like:
(.) :: (c -> d) -> (a -> b -> c) -> a -> b -> d
A concrete example: calculating difference-squared. We could write…
jameshfisher
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49
votes
2 answers
What does (f .) . g mean in Haskell?
I have seen a lot of functions being defined according to the pattern (f .) . g. For example:
countWhere = (length .) . filter
duplicate = (concat .) . replicate
concatMap = (concat .) . map
What does this mean?
Aadit M Shah
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47
votes
1 answer
function composition with reverse syntax
If I want to apply f first and g second, I have to write:
g . f
Is there another standard syntax that would allow me to write the functions in the reverse order?
f g
I know I can just invent my own syntax:
compose f g x = g (f x)
and…
fredoverflow
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45
votes
5 answers
What is happening when I compose * with + in Haskell?
I'm trying to understand the result of
(*) . (+)
in Haskell. I know that the composition operator is just the standard composition of mathematical functions- so
(f . g) = f (g x)
But:
(*) . (+) :: (Num (a -> a), Num a) => a -> (a -> a) -> a ->…
user2666425
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38
votes
3 answers
Haskell Monad bind operator confusion
Okay, so I am not a Haskell programmer, but I am absolutely intrigued by a lot of the ideas behind Haskell and am looking into learning it. But I'm stuck at square one: I can't seem to wrap my head around Monads, which seem to be fairly fundamental.…
Ord
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33
votes
1 answer
Why is function composition in Haskell right associative?
Mathematically the function composition operation is associative. Hence:
f . (g . h) = (f . g) . h
Thus the function composition operation may be defined to be either left associative or right associative.
Since normal function application in…
Aadit M Shah
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32
votes
4 answers
How to multiply functions in python?
def sub3(n):
return n - 3
def square(n):
return n * n
It's dead easy to compose functions in python:
>>> my_list
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> [square(sub3(n)) for n in my_list]
[9, 4, 1, 0, 1, 4, 9, 16, 25, 36]
Unfortunately, when…
wim
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32
votes
14 answers
Composing functions in python
I have an array of functions and I'm trying to produce one function which consists of the composition of the elements in my array.
My approach is:
def compose(list):
if len(list) == 1:
return lambda x:list[0](x)
list.reverse()
…
Starless
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31
votes
6 answers
Elegant way to combine multiple filtering functions in Haskell
Given the following filtering functions as unary predicates,
f1 :: Int -> Bool
f1 x = x > 30
f2 :: Int -> Bool
f2 x = x < 60
f3 :: Int -> Bool
f3 x = x `mod` 3 == 0
I'd like to filter a list of integers through all of them. Currently I'm doing…
Jivan
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25
votes
6 answers
What does a fullstop or period or dot (.) mean in Haskell?
I really wish that Google was better at searching for syntax:
decades :: (RealFrac a) => a -> a -> [a] -> Array Int Int
decades a b = hist (0,9) . map decade
where decade x = floor ((x - a) * s)
…
Casebash
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24
votes
1 answer
Understanding `andThen`
I encountered andThen, but did not properly understand it.
To look at it further, I read the Function1.andThen docs
def andThen[A](g: (R) ⇒ A): (T1) ⇒ A
mm is a MultiMap instance.
scala> mm
res29:…
Kevin Meredith
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