swift - constant 'result' inferred to have type (), which may be unexpected -

@ibaction func operate(sender: uibutton) {       if let operation = sender.currenttitle {          if let result = brain.performoperation(operation) {              displayvalue   = result          }          else {             displayvalue = 0.0          }      } } 

i new coding pardon coding format , other inconsistencies. have been trying out ios 8 intro swift programming taught stanford university , have ran problem modified calculator.

i 3 errors. first 1 swift compiler warning - @

if let result = brain.performoperation(operation) 

it says

constant 'result' inferred have type () may unexpected.

it gives me suggestion ----

if let result: () = brain.performoperation(operation) 

the other 2 errors

bound value in conditional binding must of optional type @ if let result line

cannot assign value of type () value of double @ "displayvalue = result"

here github link if needs more information on code.

thanks in advance.

guessing errors, expect performoperation() supposed return double? (optional double) while if fact returns nothing.

i.e. it's signature probably:

func performoperation(operation: string) {     // ... } 

.. while in fact should be:

func performoperation(operation: string) -> double? {     // ... } 

reason why think line: if let result = brain.performoperation(operation) call "unwrapping optional" , expects assigned value optional type. later assign value unwrap variable seems of double type.

by way, shorter (and more readable) way write same is:

displayvalue = brain.performoperation(operation) ?? 0.0