Type inference for return types of longer functions

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If your function is really short (exactly one expression), you can use expression body syntax. In this case, return type may be omited. 

fun sum(a : Int, b : Int) = a + b

fun sum(a : Int, b : Int) = if (a > b) a - b else (a + b)

Return type must be provided in two cases:

  • function is public or protected, i. e. it is part of API, and you don't want it's signature to change implicitly;
  • function has block body, and it may be hard to infer return types by compiler and human reading the code :)

If function is declared with block body, “return” is always neccessary, if it has expression body, “return” should be skipped.

Side note: for function literals (a.k.a. lambdas) it is a bit different, because they are one-time functions and syntax is needed to be really short:

  1. Parameter types are usually inferred.
  2. Body may contain several statements/expressions, the last one is considered as return value. “Return” is not necessary (if you don’t mean non-local return)
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