[Mojo]: Simple python like function crashes REPL

[cheburakshu]

Issue description

vscode ➜ ~/llama2.mojo (master) $ mojo -v
mojo 0.2.1 (64d14e85)
vscode ➜ ~/llama2.mojo (master) $ mojo
Welcome to Mojo! 🔥
Expressions are delimited by a blank line.
Type `:mojo help` for further assistance.
1> def a(*b):
2.     ...
3. 
lldb: /__w/modular/modular/third-party/llvm-project/llvm/include/llvm/Support/Casting.h:572: decltype(auto) llvm::cast(From &) [To = M::KGEN::PointerType, From = mlir::Type]: Assertion `isa<To>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
[14554:14554:20230914,230825.266481:ERROR file_io_posix.cc:144] open /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq: No such file or directory (2)
[14554:14554:20230914,230825.266757:ERROR file_io_posix.cc:144] open /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq: No such file or directory (2)
[User] error: The Mojo REPL has crashed and attempted recovery. If the REPL behaves inconsistently, please restart to ensure correct behavior.

expression failed to parse (no further compiler diagnostics)
1> 

[Nusab19]

try adding %python in the top ( line 1 )

Not sure if it'll work ( cuz I wasn't able to install mojo ), but saw this in the examples

[cheburakshu]

this is also native mojo code, not just python (note def requires no typing), should ideally work.

[walter-erquinigo]

I was able to reproduce this error. It seems that the compiler is not handling correctly the vararg in the function declaration.

[Mogball]

Thanks for filing! Seems like a parser crash. @laszlokindrat can you take a look?

[laszlokindrat]

Thank you for opening this issue! This is a known limitation of the current system, but the compiler shouldn't have crashed; in the next release, it will raise an error message instead.

The workaround currently is to pass variadic arguments of dynamic types as borrowed:

def a(borrowed *b): pass

[cheburakshu]

Can you make print friendly to developers? It is simply too hard to use it without having to refer to docs?

Attempt 1: Just print a.

4> def foo(borrowed *a):
5.     print(a)
6. 
[User] error: Expression [4]:6:10: no matching function in call to 'print': 
    print(a)
    ~~~~~^~~

Expression [0]:1:1: candidate not viable: callee expects 0 arguments, but 1 was specified
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'DType'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'String'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'StringRef'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'StringLiteral'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'Bool'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'FloatLiteral'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'Int'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 2 input parameters but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 2 input parameters but 0 were provided (4 more notes omitted.)
from memory.unsafe import Pointer
^


expression failed to parse (no further compiler diagnostics)
4> 

Attempt 2: Print length of a

6> def foo(borrowed *a):
7.     print(len(a))
8. 
[User] error: Expression [5]:6:14: no matching function in call to 'len': 
    print(len(a))
          ~~~^~~

Expression [0]:1:1: candidate not viable: callee expects 2 input parameters but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 2 input parameters but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 1 input parameter but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 1 input parameter but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'String'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'StringRef'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: argument #0 cannot be converted from 'variadic<pointer<!kgen.declref<_"$builtin"::_"$object"::_object>>>' to 'StringLiteral'
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 2 input parameters but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 1 input parameter but 0 were provided
from memory.unsafe import Pointer
^

Expression [0]:1:1: candidate not viable: callee expects 1 input parameter but 0 were provided (3 more notes omitted.)
from memory.unsafe import Pointer
^


expression failed to parse (no further compiler diagnostics)
6> 

[laszlokindrat]

We have an issue related this: #793 and we are working on making variadics more ergonomic! The closest thing you can do right now is something like this:

def foo(borrowed *a: Int):
    print("number of variadic args:", len(VariadicList(a)))
    for i in range(len(VariadicList(a))):
        print("arg #" + String(i) + ":", a[i])