What does a fullstop or period or dot (.) mean in Haskell?
Solution 1
f(g(x))
is
in mathematics : f ∘ g
(x)
in haskell : ( f . g )
(x)
Solution 2
It means function composition. See this question.
Note also the f.g.h x
is not equivalent to (f.g.h) x
, because it is interpreted as f.g.(h x)
which won't typecheck unless (h x) returns a function.
This is where the $ operator can come in handy: f.g.h $ x
turns x from being a parameter to h
to being a parameter to the whole expression. And so it becomes equivalent to f(g(h x))
and the pipe works again.
Solution 3
.
is a higher order function for function composition.
Prelude> :type (.)
(.) :: (b -> c) -> (a -> b) -> a -> c
Prelude> (*2) . (+1) $ 1
4
Prelude> ((*2) . (+1)) 1
4
Solution 4
"The period is a function composition operator. In general terms, where f and g are functions, (f . g) x means the same as f (g x). In other words, the period is used to take the result from the function on the right, feed it as a parameter to the function on the left, and return a new function that represents this computation."
Source: Google search 'haskell period operator'
Solution 5
It is a function composition: link
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Casebash
Bachelor of Science (Adv Maths) with Honors in Computer Science from University of Sydney Programming C/C++/Java/Python/Objective C/C#/Javascript/PHP
Updated on April 16, 2022Comments
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Casebash about 2 years
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) s = 10 / (b - a)
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kennytm about 14 yearsA period can also be a namespace separator (e.g.
Data.Vector.Unboxed.length
). -
Antal Spector-Zabusky about 14 yearsFor searching for information about Haskell code, I heartily recommend Hoogle (haskell.org/hoogle), a search engine for types (e.g. searching for
(a -> b) -> [a] -> [b]
turns upmap
) and function/operator names (so searching formap
turns upmap
, and searching for.
turns up the Prelude function composition operator(.)
). There's also Hayoo! (holumbus.fh-wedel.de/hayoo/hayoo.html), which has less of an emphasis on types but indexes more packages. -
Don Stewart about 13 yearspossible duplicate of Dot Operator in Haskell: need more explanation
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Alex Jenter about 14 yearsYou just need to remember that the function application operator (space) has the highest priority. After some time it will all make sense.
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Sid over 6 yearsBetter suited as a comment.