Haskell: Define length function using map and sum

There are many ways to do this.

Way 1

Prelude library defines const function like below.

const x _ = x

Whatever the second argument, const function always return the first argument of const function.

Example, We can implment length function like below.

Prelude> let length = sum.map(const 1)
Prelude> 
Prelude> length []
0
Prelude> length [1, 2, 3]
3
Prelude> length "Hello World"
11

Way 2

Following statement works for list of integers.

sum ( map (+1) list) - sum ( map (+0) list)

Prelude> let list = [2, 3, 4, 5]
Prelude> 
Prelude> sum ( map (+1) list) - sum ( map (+0) list)
4

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