fansh> alterList := |Int[] v -> Int[]| { v = [1]; return v; }
|sys::Int[]->sys::Int[]|

fansh> l1 := Int[,]
[,]

fansh> l2 := alterList2(l1)
[1]

fansh> l1
[,]

fansh> alterStr := |Str s->Str| { s = "altered"; return s; }
|sys::Str->sys::Str|

fansh> s1 := "test"
test

fansh> s2 := alterStr(s1)
altered

fansh> s1
test

Hi guys, based on codes above, seems like List and Str are passed into func/method as value and not reference. Am I correct? If I am, why is it so? I can't seem to find any explanation in the docs.

So if I want to replace the value of the original variable like in Java/C#, how should I do it idiomatically in Fantom?

Str's are immutable (like in Java) and Lists are passed by ref. What you're doing is re-assigning your local variables v and s and returning them - you're not changing the refs to l1 and s2

if I want to replace the value of the original variable

l1 := Int[,]
l1 = Int[1]

s1 := "test"
s1 = "altered"

This code:

alterList := |Int[] v -> Int[]| { v = [1]; return v; }

You are creating a new Int[] and assigning it to v. You probably want something like this if you want to modify in place:

alterList := |Int[] v -> Int[]| { v.add(1); return v; }

Ah, I see it now. I think I got it mixed up with out parameter in C#.

Thanks guys!