Question

In Perl 5, we can apply functional programming techniques (using closures, higher order functions like map, grep, etc.). But how about function composition? Let’s say, in Haskell it can be done very easily with (.) function:

map (negate . abs) [-3, 2, 4, -1, 5]

What would be the equivalent of such “dot function” in Perl?

Answer 1

Sadly, I don’t know Haskell.

But function composition essentially is putting the output of one function into the next function as an argument.

output = (negate . abs)(input) is the same as output = negate(abs(input)). In Perl, parens are often optional, and the input is implicit in the map function, so we can just say

output = map (negate abs) list

Now just translate this to Perl syntax, and we have

my @output = map {- abs} (1,2,3);

for the mathematical/algebraic negation, and

my @output = map {! abs} (1,2,3);

for the logical negation (which is the same as map {! $_} (1,2,3), of course).