swift - How to handle initial nil value for reduce functions -

i learn , use more functional programming in swift. so, i've been trying various things in playground. don't understand reduce, though. basic textbook examples work, can't head around problem.

i have array of strings called "todoitems". longest string in array. best practice handling initial nil value in such cases? think happens often. thought of writing custom function , use it.

func optionalmax(maxsofar: int?, newelement: int) -> int {     if let definitemaxsofar = maxsofar {         return max(definitemaxsofar, newelement)     }     return newelement }     // testing - nums array of ints. works. var maxvalueofints = nums.reduce(0) { optionalmax($0, $1) }     // error: cannot invoke 'reduce' argument list of type ‘(nil, (_,_)->_)' var longestofstrings = todoitems.reduce(nil) { optionalmax(count($0), count($1)) } 

it might swift not automatically infer type of initial value. try making clear explicitly declaring it:

var longestofstrings = todoitems.reduce(nil int?) { optionalmax($0, count($1)) } 

by way notice not count on $0 (your accumulator) since not string optional int int?

generally avoid confusion reading code later, explicitly label accumulator a , element coming in serie x:

var longestofstrings = todoitems.reduce(nil int?) { a, x in optionalmax(a, count(x)) } 

this way should clearer $0 , $1 in code when accumulator or single element used.

hope helps