Selecting a function from a list of functions in Golang

Question

Basically if I have a slice or array of any arbitrary functions, how can I select only the ones that return int, or select only the ones that take ints?

I figured that I would need to use the reflect package, but just reading the docs didn't really help me figure out exactly how to do it.

Theft

See if this thread on golang-nuts helps – Dharana

can you explain the problem you are trying to solve instead of the solution you are trying to implement? There is likely an easier way to do this in Go. The type of approach you are attempting is very awkward in Go. Depending on what you're trying to do, dystroy's suggestion may be your only option, but there's a very good chance you can do something far less confusing. – Calcimine

Nope, what I am trying to do is in fact confusing. Consider creating a graph where each node is a function. I want to search over the space of graphs given an arbitrary list of functions. The idea would be that it can be used as a function approximator ala Machine Learning. I believe that dystroy's solution is what I want. If you want to suggest a language in which it would be nicer I am open to ideas. I have heard that C# could do it. – Theft

ah, I don't have ML experience to advise, but it sounds like you can get away with either using a function type as a field, e.g. type Node struct { Neighbors []*Node; Fn func(int...) int}, or using a recursive interface like type Node interface { Children []Node; Fn(int...) int }. There's probably a way to avoid reflect that will save you some trouble. Rob Pike's talk "Lexical Scanning in Go" discusses the recursive interface thing. It's very useful. – Calcimine

I will look at that talk, but the hard part is that I want to make this general enough to work with any arbitrary function and automatically figure out where it can fit in the graph it is building. If I did what it appears you are suggesting then it would basically be another variant of a Neural Network. – Theft

you were right: that is, in fact, confusing. Anywho, function types can have methods on them in Go, which means that an interface{} value can contain a function type (because a function type still has a method set, even if it is empty). If you define a type Node struct { F interface{} } and stick functions in it, you can make this a little easier to maintain: play.golang.org/p/ANJBr1i8Zh. Cool problem. – Calcimine

Answer 1

This program prints the functions taking an int as parameter or returning an int :

package main

import (
    "fmt"
    "reflect"
)

func main() {
    funcs := make([]interface{}, 3, 3) // I use interface{} to allow any kind of func
    funcs[0] = func (a int) (int) { return a+1} // good
    funcs[1] = func (a string) (int) { return len(a)} // good
    funcs[2] = func (a string) (string) { return ":("} // bad
    for _, fi := range funcs {
        f := reflect.ValueOf(fi)
        functype := f.Type()
        good := false
        for i:=0; i<functype.NumIn(); i++ {
            if "int"==functype.In(i).String() {
                good = true // yes, there is an int among inputs
                break
            }
        }
        for i:=0; i<functype.NumOut(); i++ {
            if "int"==functype.Out(i).String() {
                good = true // yes, there is an int among outputs
                break
            }
        }
        if good {
            fmt.Println(f)
        }
    }
}

I think the code is self explanatory

Eupatrid