With

reduce

accumulator type is fixed, it is always the same type as the element type.

If for example you want to find the max value in an

IntArray

, you could use

reduce

because you are reducing to the same type (

Int

)

If instead you wanted to count how many times each value occurs in the

IntArray

, you would have to use

fold

cause you are reducing to a

Map<Int, Int>

(or similar structure) and not just simply an

Int

so

fold

scans through your collection and appends each value to the accumulator,

reduce

picks any two adjacent values and applies the operation >>replacing<< (but not really) the values with the result then repeats. The difference is that

reduce

can be done in parallel,

fold

not really.

simply speaking

fold

is a line of operations,

reduce

is a tree