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I'm trying to do find a way to do something like this:
(head, last) `someFunction` [1, 2, 3]
to produce the tuple (1, 3) as output.
It seems similar in theory to an applicative functor, but a little backwards. I'm guessing there's a similar function that does this (or some way to make one), but I can't seem to find it/figure it out.
I tried defining a function like this:
fmap' :: ((a -> b), (a -> b)) -> [a] -> (b, b)
fmap' (f1, f2) xs = (f1 xs, f2 xs)
but GHC won't actually compile this.
Any help would be great; thanks!
Edit (a whole year later!):
My fmap' wouldn't compile because the type signature was wrong. Obviously there are better ways to do what I was doing, but the type of my fmap' should instead be:
fmap' :: ((a -> b), (a -> b)) -> a -> (b, b)
In that case, it compiles and runs just fine.
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Maps or hash tables are collections of key value pairs. Tuples are an immutable sequence of values and are used for aggregating values.
map takes a function and a list and applies that function to every element in the list, producing a new list.
A tuple is a sequence of values. The values can be of any type, and they are indexed by an integer, so tuples are not like lists. Tuples are immutable which means you cannot add more elements to the tuple as the program runs.
Use parentheses and commas to create tuples. Use one comma to create a pair. Use more commas to create tuples with more components. Note that it is also possible to declare tuples using in their unsugared form.
I think you can do this with arrows.
head &&& last $ [1,2,3]
will return (1,3).
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