parenthesis in Haskell functions - haskell

I just want to know how do we know which functions need brackets () and which ones do not? For example
replicate 100 (product (map (*3) (zipWith max [1,2,3,4,5] [4,5,6,7,8])))
works fine. But
replicate 100 (product (map (*3) (zipWith (max [1,2,3,4,5] [4,5,6,7,8]))))
does not work. It is because I put a set of brackets for zipWith. In this small example, zipWith and max do not have brackets, but replicate, product and map do. In general is there a way to know/figure out which functions need brackets and which ones dont.

Function application is left associative. So, when you write an expression like:
f g h x
it means:
((f g) h) x
And also the type of zipWith provides a clue:
zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
it says that zipWith has 3 parameters: a function and two lists.
When you write:
zipWith (max [1,2,3,4,5] [4,5,6,7,8])
The interpreter will understand that
max [1,2,3,4,5] [4,5,6,7,8]
will be the first parameter to zipWith, which is type incorrect. Note that zipWith expects a function of two arguments as its first argument and, as pointed out by #Cubic, max [1,2,3,4,5] [4,5,6,7,8] will return the maximum
between these two lists according the usual lexicographic order, which will be of type [a], for some type a which is instance of Ord and Num. Said that, the error become evident since you are trying to pass a value of type
(Num a, Ord a) => [a]
where a value of type
(a -> b -> c)
is expected.

Rodrigo gave the right answer. I'll just add that it is a misconception to think that some functions need parentheses, while others don't.
This is just like in school math:
3 * (4+5)
It is simply not the case that + expressions need parentheses and * expressions don't need them in general.
In Haskell, you can always get away without parentheses at all. Whenever you need to enclose an expression in parentheses, the alternative is to introduce a local name and bind it to that expression, then use the name instead of the expression.
In your example:
replicate 100 (product (map (*3) (zipWith max [1,2,3,4,5] [4,5,6,7,8])))
let list1 = product list2
list2 = map thrice list3
thrice x = x*3
list3 = zipWith max [1,2,3,4,5] [4,5,6,7,8]
in replicate 100 list1
In fact, I often write functions top down thus:
foo x y z = result
where
result = ...
...
However, as it was said before, expressions that consist of function applications can also often be written without parentheses by making use of (.) and ($) and in such cases, the top down approach from above may be overly verbose and the following would be much clearer (because there is no noise through newly introduced names):
replicate 100
. product
. map (*3)
$ zipWith max [1..5] [4..8]

Related

Haskell Functions (map,foldr, foldl)

I am struggling to think of a way to utilize these functions for this beginner level coding class that I am taking to learn functional programming in Haskell. The functions I have to write are shown below, asum is supposed to turn a list of integers [a1,a2,..,an] into the alternating sum a1-a2+a3-a4+.… and I am not sure how to approach it with these functions. The xor function is supposed to that computes the XOR of a list of Booleans. I need some help to understand how to use these functions and it would greatly appreciated. I am also new to Haskell so any explanations would help. Thanks I have to use map foldr foldl.
asum :: (Num a) => [a] -> a
xor :: [Bool] -> Bool
I would say start by running the following, one by one, in GHCi:
:t foldr
:info foldr
:doc foldr
:t foldl
:info foldl
:doc foldl
:t map
:info map
:doc map
Or better, open hoogle.haskell.org and search each of the above mentioned functions and click on the first link.
But I agree that Haskell documentation are difficult to read, especially for beginners. I'm a beginner and I have a lot of difficulty reading and understanding them.
Here's a function that uses map and foldr to show how foldr works:
printFoldr xs = foldr (\x acc -> "(" ++ x ++ " + " ++ acc ++ " )") "0" $ map show xs
Now running watch this:
printFoldr [1..5]
-- outputs the following:
"(1 + (2 + (3 + (4 + (5 + 0 ) ) ) ) )"
This shows us how foldr is evaluated. Before going into how foldr is evaluated, let's look briefly at map.
map show [1..5]
-- outputs the following:
["1","2","3","4","5"]
This means that map takes 2 arguments. A list and a function that is applied to each element of the list. The result is a new list with the function applied to each element. Thus, applying show to each number outputs their string representation.
Back to foldr. foldr takes 3 arguments:
a function of type a -> b -> b
an initial value of type b
a list of type [a]
foldr takes each and every value of the provided list and applies this function to it. What is special is that map retains the output of the function over each iteration and passes it to the function as its second argument on the next run. Therefore it is convenient to write the function that is passed foldr as follows: (\el acc -> do something). Now on the next iteration of foldr, acc will hold the value of the previous run and el will be the current element from the list. BTW, acc stands for accumulator and el for element. This enables us to reduce elements of the provided list to something completely new.
As you can see in printFoldr, the initial value is just an empty string but it gradually adds the lists elements to it showing how it would have reduced the elements of the list to their sum.
Here's an idea:
a1-a2+a3-a4+...
=
a1-(a2-(a3-(a4-(...(an-0)...))))
This fits pretty well to the foldr pattern of recursion,
foldr f z [a1,a2,a3,a4,...,an]
=
a1`f`(a2`f`(a3`f`(a4`f`(...(an`f`z)...))))
So it can be coded by setting f = ... and z = ... and calling
asum :: (Num a) => [a] -> a
asum xs = foldr f z xs
where
f = (...)
z = (...)
You will need to complete this definition.
For the XOR of a list of Booleans, assuming it is to be True if one and only one of them is True, and False otherwise, we can imagine this sequence of transformations:
[ True, False, False, True, True, False, ...]
==>
[ t, f, f, t, t, f, ...]
where t and f are some specially chosen numbers. And then we can find the sum of this second list (not alternating sum, just a sum of a list of numbers) and check whether it is equal to ... some (other?) special number, let's call it n1:
xor :: [Bool] -> Bool
xor bools = (aNumber ... n1)
where
list1 = bools
list2 = fun1 transform list1
transform False = f
transform True = t
f = ...
t = ...
aNumber = sum list2
n1 = ...
fun1 = ...
sum listOfNums = ...
fun1 is the function which transforms each element of its argument list according to the given function, called transform above. It is one of the two functions left from the three you were given, considering we've already been using foldr.
sum is to be implemented by using the last function that's left.
FYI,
map foo [a1,a2,a3,...,an]
=
[foo a1, foo a2, foo a3, ..., foo an]
and
foldl f z [a1,a2,a3,...,an]
=
((((z`f`a1)`f`a2)`f`a3)...)`f`an

Haskell: Purpose of the flip function?

I am a bit surprised that this was not asked before. Maybe it is a stupid question.
I know that flip is changing the order of two arguments.
Example:
(-) 5 3
= 5 - 3
= 2
flip (-) 5 3
= 3 - 5
= -2
But why would I need such a function? Why not just change the inputs manually?
Why not just write:
(-) 3 5
= 3 - 5
= -2
One is unlikely to ever use the flip function on a function that is immediately applied to two or more arguments, but flip can be useful in two situations:
If the function is passed higher-order to a different function, one cannot simply reverse the arguments at the call site, since the call site is in another function! For example, these two expressions produce very different results:
ghci> foldl (-) 0 [1, 2, 3, 4]
-10
ghci> foldl (flip (-)) 0 [1, 2, 3, 4]
2
In this case, we cannot swap the arguments of (-) because we do not apply (-) directly; foldl applies it for us. So we can use flip (-) instead of writing out the whole lambda \x y -> y - x.
Additionally, it can be useful to use flip to partially apply a function to its second argument. For example, we could use flip to write a function that builds an infinite list using a builder function that is provided the element’s index in the list:
buildList :: (Integer -> a) -> [a]
buildList = flip map [0..]
ghci> take 10 (buildList (\x -> x * x))
[0,1,4,9,16,25,36,49,64,81]
Perhaps more frequently, this is used when we want to partially apply the second argument of a function that will be used higher-order, like in the first example:
ghci> map (flip map [1, 2, 3]) [(+ 1), (* 2)]
[[2,3,4],[2,4,6]]
Sometimes, instead of using flip in a case like this, people will use infix syntax instead, since operator sections have the unique property that they can supply the first or second argument to a function. Therefore, writing (`f` x) is equivalent to writing flip f x. Personally, I think writing flip directly is usually easier to read, but that’s a matter of taste.
One very useful example of flip usage is sorting in descending order. You can see how it works in ghci:
ghci> import Data.List
ghci> :t sortBy
sortBy :: (a -> a -> Ordering) -> [a] -> [a]
ghci> :t compare
compare :: Ord a => a -> a -> Ordering
ghci> sortBy compare [2,1,3]
[1,2,3]
ghci> sortBy (flip compare) [2,1,3]
[3,2,1]
Sometimes you'll want to use a function by supplying the second parameter but take it's first parameter from somewhere else. For example:
map (flip (-) 5) [1..5]
Though this can also be written as:
map (\x -> x - 5) [1..5]
Another use case is when the second argument is long:
flip (-) 5 $
if odd x
then x + 1
else x
But you can always use a let expression to name the first parameter computation and then not use flip.

Haskell code explanation

xs = [1,2,3]::[Float]
ys = map (+) xs
This was a question in an old test and there is no solution sheet.
The questions:
1) What kind of signature does ys have?
2) Explain why and draw how ys looks like
For the first question I know that xs is of type float and so should ys(I run the program in ghci too).
As for the second one I have no idea, because when I run the code nothing happens. When I run it and the run ys on a separate row I get an error.
Can someone help me with a hint?
For the first question I know that xs is of type float
er, no. xs has type [Float]: a list of floats.
and so should ys
ys does not have the same type as xs. You probably think so because you've read that + requires the arguments and result to have the same type:
(+) :: Num a => a -> a -> a
...or if you instantiate it to Float numbers
(+) :: Float -> Float -> Float
This is correct, nevertheless (+) is not an endomorphism (a function mapping a type to itself, as it would have to be if ys was the same type as xs) because it has two number arguments.
With map (+) you're considering (+) as a function of a single argument, not of two arguments. In most programming languages this would actually be an error, but not so in Haskell: in Haskell, all functions actually have only one argument. Functions with “multiple arguments” are really just functions on interesting types, that make it seem as if you're passing multiple arguments. In particular, the signature of (+) is actually shorthand for:
(+) :: Float -> (Float -> Float)
So, considered as a one-argument function, (+) actually maps numbers to number-endomorphisms. Hence,
map (+) :: [Float] -> [Float -> Float]
and
ys :: [Float -> Float]
– a list of number-functions. Specifically, it's this list:
ys = [(+) 1 , (+) 2 , (+) 3 ]
≡ [(1+) , (2+) , (3+) ]
≡ [\n -> 1+n, \n -> 2+n, \n -> 3+n]
I could, for example, use it like this:
GHCi> let [f,g,h] = ys in [f 3, g 2, h 1]
[4,4,4]
GHCi> map ($ 10) ys -- applies all functions separately to the number 10
[11,12,13]
GHCi> foldr ($) 0 ys -- applies all the functions one after another to 0
6
BTW, IMO you're asking the question the wrong way around. In Haskell, you don't want to consider some code and wonder what type it has – that is more an ML or even Lisp approach. I'd always start with the type signature, and work out the implementation “outside to in” (typed holes are very handy for this). This possibility is one of the big advantages of functional programming in comparison to procedural languages.
I don't have ghci at the moment, apologies if something I say is wrong.
xs is type [Float] and ys is of type [Float -> Float](it's a list of functions that each take a Float and return a Float). ys will be [(+) 1, (+) 2, (+) 3] because map applies (+) to each elements in xs. But you cannot print ys because functions do not derive Show
ys type is [Float -> Float], a list of functions that receive a number return the number +1 (first elem), the number + 2 (the second) and the number +3 (the last).
Please, bear in mind that + is a is applied with a single argument for each list element so it does return another function.
If you wanted to add all the items in the List, you should use a reduce function, such as foldl.
let zs = foldl (+) 0 xs
I hope this helps.
Cristóbal

What does foldr do in Haskell? [duplicate]

This question already has answers here:
How does foldr work?
(11 answers)
Closed 6 years ago.
So I came across the foldr function in Haskell which from what I pick up, you can use to calculate the product and sum of a list:
foldr f x xs
foldr (*) 1 [1..5] = 120
foldr (+) 0 [1..5] = 15
And adding numbers onto the x part would like, add on to the overall sum or multiply onto the final product
What does foldr actually do and why would anyone use it instead of the built in functions 'sum' or 'product' etc?
sum and product are themselves defined using fold (foldl, not foldr, but let's set aside that distinction for now), so in a sense you are using fold when using those functions. Likewise or, and, concat and many more are all defined using folds as well. So that's already one reason for folds to exist: many of the standard functions can be defined using them instead of having redundant code.
So when would you use folds directly? When doing something that there isn't already a specific function for, i.e. when you want to combine the elements of the list using something other than + or * (or ||, && or ++).
Say you have a list of single-digit numbers and you want to "concatenate" them into one number:
concatDigits = foldl (\acc d -> d + acc * 10) 0
Now concatDigits [1,2,3] gives you 123.
Or you've defined some datastructure and you want to convert lists to it:
fromList = foldr insert empty`
In fact that's how fromList is commonly defined for many data structures.
The Wikipedia entry has a good illustration of what foldr does to a list and how it differs from foldl.
The list [1,2,3] is generated by the cons operator (:) and the empty list [] by this expression tree:
:
/ \
1 :
/ \
2 :
/ \
3 []
foldr f z replaces the cons operator nodes with f and the empty list with z:
By contrast, foldl f z reorients the expression tree and repositions the zero element at the opposite end of the computation:
Some observations:
Note that foldr (:) [] leaves a list unchanged:
foldr (:) [] [1..10] == [1..10]
This makes sense since we are simply replacing the cons operator with itself and the empty list with the empty list and thus not changing anything.
If we replace the empty list with some other list we should be able to append two lists, e.g.:
foldr (:) [9,8,7] [1,2,3] == [1,2,3,9,8,7]
The cons operator may be thought of a way to add an element to a list:
(:) :: a -> [a] -> [a]
If we define f as:
f :: Int -> [Int] -> [Int]
f a as = [0] ++ [a] ++ as
then foldr f [] will prepend a 0 before each element of the input list:
foldr f [] [1,2,3] == [0,1,0,2,0,3]
Data.Set has a function insert whose signature is structually similar to that of (:):
insert :: Ord a => a -> Set a -> Set a
Indeed, we can convert a list to a Set using foldr
import qualified Data.Set as S
foldr S.insert S.empty [1,2,3]
Note how the empty list is replaced with the empty set - S.empty.
This is the idiomatic approach to building up data structures one element at a time - e.g. hash maps, tries, trees, etc.

Composing a chain of 2-argument functions

So I have a list of a functions of two arguments of the type [a -> a -> a]
I want to write a function which will take the list and compose them into a chain of functions which takes length+1 arguments composed on the left. For example if I have [f,g,h] all of types [a -> a -> a] I need to write a function which gives:
chain [f,g,h] = \a b c d -> f ( g ( h a b ) c ) d
Also if it helps, the functions are commutative in their arguments ( i.e. f x y = f y x for all x y ).
I can do this inside of a list comprehension given that I know the the number of functions in question, it would be almost exactly like the definition. It's the stretch from a fixed number of functions to a dynamic number that has me stumped.
This is what I have so far:
f xs = f' xs
where
f' [] = id
f' (x:xs) = \z -> x (f' xs) z
I think the logic is along the right path, it just doesn't type-check.
Thanks in advance!
The comment from n.m. is correct--this can't be done in any conventional way, because the result's type depends on the length of the input list. You need a much fancier type system to make that work. You could compromise in Haskell by using a list that encodes its length in the type, but that's painful and awkward.
Instead, since your arguments are all of the same type, you'd be much better served by creating a function that takes a list of values instead of multiple arguments. So the type you want is something like this: chain :: [a -> a -> a] -> [a] -> a
There are several ways to write such a function. Conceptually you want to start from the front of the argument list and the end of the function list, then apply the first function to the first argument to get something of type a -> a. From there, apply that function to the next argument, then apply the next function to the result, removing one element from each list and giving you a new function of type a -> a.
You'll need to handle the case where the list lengths don't match up correctly, as well. There's no way around that, other than the aforementioned type-encoded-lengths and the hassle associate with such.
I wonder, whether your "have a list of a functions" requirement is a real requirement or a workaround? I was faced with the same problem, but in my case set of functions was small and known at compile time. To be more precise, my task was to zip 4 lists with xor. And all I wanted is a compact notation to compose 3 binary functions. What I used is a small helper:
-- Binary Function Chain
bfc :: (c -> d) -> (a -> b -> c) -> a -> b -> d
bfc f g = \a b -> f (g a b)
For example:
ghci> ((+) `bfc` (*)) 5 3 2 -- (5 * 3) + 2
17
ghci> ((+) `bfc` (*) `bfc` (-)) 5 3 2 1 -- ((5 - 3) * 2) + 1
5
ghci> zipWith3 ((+) `bfc` (+)) [1,2] [3,4] [5,6]
[9,12]
ghci> getZipList $ (xor `bfc` xor `bfc` xor) <$> ZipList [1,2] <*> ZipList [3,4] <*> ZipList [5,6] <*> ZipList [7,8]
[0,8]
That doesn't answers the original question as it is, but hope still can be helpful since it covers pretty much what question subject line is about.

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