How to declare a function type in a trait?

This is a simple class in Python

class MyPair:
    def __init__(first, second):
        self.first = first 
        self.second = second

        def _hello():
            return None 
        
        self._hi = _hello

class MyPair:
    def __init__(first, second):
        self.first = first 
        self.second = second

        def _hello():
            return None 
        
        self._hi = _hello

Now I thought I could this in Mojo in someway but stucked

struct MyPair:
    var first: Int
    var second: Int
    var _hi 

    fn __init__(inout self, first: Int, second: Int):
        self.first = first
        self.second = second
        fn _hello():
            return None 
        
        self._hi = _hello

struct MyPair:
    var first: Int
    var second: Int
    var _hi 

    fn __init__(inout self, first: Int, second: Int):
        self.first = first
        self.second = second
        fn _hello():
            return None 
        
        self._hi = _hello

Is there any possible way of doing this in Mojo? Error:

error: Expression [17] wrapper:22:13: struct field declaration must have a type
    var _hi 
            ^
error: Expression [17]:12:13: 'MyPair' value has no attribute '_hi'
        self._hi = _hello

error: Expression [17] wrapper:22:13: struct field declaration must have a type
    var _hi 
            ^
error: Expression [17]:12:13: 'MyPair' value has no attribute '_hi'
        self._hi = _hello

9 Replies

mad alex 1997•3mo ago

You have to set the type to match the function signature. And Fn functions must explicitly return there type.

struct MyPair:
    var first: Int
    var second: Int
    var _hi: fn()->None 

    fn __init__(inout self, first: Int, second: Int):
        self.first = first
        self.second = second
        fn _hello()->None:
            return None 
        
        self._hi = _hello

struct MyPair:
    var first: Int
    var second: Int
    var _hi: fn()->None 

    fn __init__(inout self, first: Int, second: Int):
        self.first = first
        self.second = second
        fn _hello()->None:
            return None 
        
        self._hi = _hello

But this makes for an inflexible _hello function as now it must return None.

ModularBot•3mo ago

Congrats @mad alex 1997, you just advanced to level 11!

Anindya•3mo ago

Thanks for the answer, so is it not possible to be more flexible with this apperoach in any ways?

mad alex 1997•3mo ago

Not without doing something weird, strong typing is a double edged sword.

Anindya•3mo ago

I see, but still it is a good start, thanks and now I am starting to see the cons of strong typings .. ughh

mad alex 1997•3mo ago

Technically if you are willing to do something verbose and your functions don't need parameters (I'll put in a bug report for it tomorrow) only arguments.

fn a()->None:
    print("a")
    return None

fn b(i:Int)->Int:
    print(i)
    return i*2

def main():
    var func: Variant[fn()->None,fn(i:Int)->Int]
    func = a
    if func.isa[fn()->None]():
        func._get_ptr[fn()->None]()[0]()
    func = b
    if func.isa[fn(i:Int)->Int]():
        var res = func._get_ptr[fn(i:Int)->Int]()[0](1)
        print(res)

fn a()->None:
    print("a")
    return None

fn b(i:Int)->Int:
    print(i)
    return i*2

def main():
    var func: Variant[fn()->None,fn(i:Int)->Int]
    func = a
    if func.isa[fn()->None]():
        func._get_ptr[fn()->None]()[0]()
    func = b
    if func.isa[fn(i:Int)->Int]():
        var res = func._get_ptr[fn(i:Int)->Int]()[0](1)
        print(res)

Anindya•3mo ago

Hi, so there was one more problem I am facing

from python import Python


struct Module:
    var func_module: PythonObject

    fn __init__(inout self, module_path: String):
        try:
            Python.add_to_path(module_path)
            self.func_module = Python.import_module("func")
        except Exception:
            pass

from python import Python


struct Module:
    var func_module: PythonObject

    fn __init__(inout self, module_path: String):
        try:
            Python.add_to_path(module_path)
            self.func_module = Python.import_module("func")
        except Exception:
            pass

So, in any how I was not able to initialize my func_module which is taken from func.py file. I got to insert it under try block other wise giving me error, and once that done, I am getting this kind of error:

/Users/anindyadeepsannigrahi/workspace/Mojo/autograd.mojo:7:8: error: 'self.func_module' is uninitialized at the implicit return from this function
    fn __init__(inout self, module_path: String):
       ^
/Users/anindyadeepsannigrahi/workspace/Mojo/autograd.mojo:7:23: note: 'self' declared here
    fn __init__(inout self, module_path: String):
                      ^
mojo: error: failed to run the pass manager

/Users/anindyadeepsannigrahi/workspace/Mojo/autograd.mojo:7:8: error: 'self.func_module' is uninitialized at the implicit return from this function
    fn __init__(inout self, module_path: String):
       ^
/Users/anindyadeepsannigrahi/workspace/Mojo/autograd.mojo:7:23: note: 'self' declared here
    fn __init__(inout self, module_path: String):
                      ^
mojo: error: failed to run the pass manager

Ryulord•3mo ago

the compiler needs to know with 100% certainty that self.func_module is initialized by the end of __init__ but it can't because it only gets initialized in your try block but not the except block. You'll need to give it some value in your except block or just get rid of the try-catch and make __init__ raise

Anindya•3mo ago

yes, I am thinking of not adding try block but then you saying just using a raise would work with an if?

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How to declare a function type in a trait?