Question about iterating a list of strings

Hi, I've got a background in Rust (10 years) and in Python (1 year) and I try to learn Mojo. I've try this :

fn main():
  try:
    var stuff = String("dddd\nffff\njjjj\nkkkk").split("\n")
    for x in stuff:
      print(x)
  except:
    print("failed")

fn main():
  try:
    var stuff = String("dddd\nffff\njjjj\nkkkk").split("\n")
    for x in stuff:
      print(x)
  except:
    print("failed")

and got this obscure error message :

/source/prog.mojo:7:12: error: no matching function in call to 'print'
      print(x)
      ~~~~~^~~
/source/prog.mojo:1:1: note: candidate not viable: expected at most 0 positional arguments, got 1
fn main():
^
/source/prog.mojo:1:1: note: candidate not viable: callee expects 2 parameters, but 0 were specified
fn main():
^
/source/prog.mojo:7:13: note: failed to infer parameter 'T', argument type 'Reference[String, 1, stuff, 0]' does not conform to trait 'Stringable'
      print(x)
            ^
mojo: error: failed to parse the provided Mojo source module

/source/prog.mojo:7:12: error: no matching function in call to 'print'
      print(x)
      ~~~~~^~~
/source/prog.mojo:1:1: note: candidate not viable: expected at most 0 positional arguments, got 1
fn main():
^
/source/prog.mojo:1:1: note: candidate not viable: callee expects 2 parameters, but 0 were specified
fn main():
^
/source/prog.mojo:7:13: note: failed to infer parameter 'T', argument type 'Reference[String, 1, stuff, 0]' does not conform to trait 'Stringable'
      print(x)
            ^
mojo: error: failed to parse the provided Mojo source module

but if I use indexes, it works :

for x in range(0, len(stuff)):
      print(stuff[x])

for x in range(0, len(stuff)):
      print(stuff[x])

I seem to have a hard time with the most basic things 🙂 What have I misunderstood ? Thanks

8 Replies

Serg Gini•4mo ago

You need to use print(x[]) for 'dereference' or something like that. I've seen it in the other code, but still don't understand this model yet by myself)

Frank Saez•4mo ago

thanks !

Melody Daniel•4mo ago

Auto-deref has been implemented and will slowly get added to the stdlib so you wouldn't need to deference pointers yourself in the future

Frank Saez•4mo ago

@FSaez good to know, thanks.

Three chickens in the green bag•4mo ago

Yes. There’s a plan for auto dereferencing. @FSaez You should open an issue for a wrong or unhelpful error message

ilrico•3mo ago

I was wondering, any particular reason to use [] operator to deference, instead of widly-accepted *, present in mainstream languages? I understand the "access to value" meaning of [], close to array[index], but asterisk is so familiar to programmers...

Melody Daniel•3mo ago

"*" is already heavily overloaded in Python Also, the square bracket acts as a getter. I believe ref[] is the same as ref[0] or something like that

ilrico•3mo ago

yes, the getter meaning is what I called "access to value". But * overloading is indeed a very valid explanation. thx

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Question about iterating a list of strings