Creating a map of functions that return an interface

Question

I have a Go package that has a large number of (auto-generated) models:

Tax2001_1
Tax2001_2
Tax2001_3
...
Tax2020_1

Each is defined like this:

func NewTax2011_1() *Tax2011_1 {
    return &Tax2011_1 { ... }
}

I want to access them depending on a value (timestamp) that is only known at runtime. So I am trying to put the model constructors into a map:

package tax

type TaxModel interface {
  Calculate()
}

var taxModels = make(map[string]func() *TaxModel)

func init() {
  ...
  taxModels["2011_1"] = NewTax2011_1
  taxModels["2011_2"] = NewTax2011_2
  ...
}

The above code is not correct:

cannot use NewTax2011_1 (type func() *Tax2011_1) as type func() *TaxModel in assignment

Any hint how to achieve that?

Burak Serdar · Accepted Answer · 2020-07-31 20:28:07Z

3

Assuming Tax2011_1 and its friends implement TaxModel interface, you can declare the constructor to return the interface:

func NewTax2011_1() TaxModel {
  return &Tax2011_1{}
}

You should not use an interface pointer:

var taxModels = make(map[string]func() TaxModel)

Then it should work.

If you cannot change the constructors, you can use an adapter function:

func NewTax2011_1() *Tax2011_1 {...}

var taxModels = make(map[string]func() TaxModel)

func init() {
  ...
  taxModels["2011_1"] = func() TaxModel {return NewTax2011_1()}
  taxModels["2011_2"] = func () TaxModel {return NewTax2011_2() }
  ...
}

answered Jul 31, 2020 at 20:28

Burak Serdar

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2 Comments

knipknap Over a year ago

Thanks! That looks like it would work - if I could change the generated packages. What if I can not?

Burak Serdar Over a year ago

Then use the suggestion in the second part.

Zan Lynx · Accepted Answer · 2020-07-31 20:33:46Z

1

Here's a test I made which seems to work. Looks like you need to not use a pointer to interface. That is what seems to be the problem.

package main

import "fmt"

type Model_1 struct {
        Name string
}

type Model_2 struct {
        Name string
}

func (m *Model_1) Calculate() int64 {
        return 0
}

func (m *Model_2) Calculate() int64 {
        return 0
}

type Model interface {
        Calculate() int64
}

func main() {
        m := make(map[string]func() Model)
        m["test"] = func() Model {
                return &Model_1{Name: "m"}
        }
        fmt.Println(m["test"]().Calculate())
}

edited Jul 31, 2020 at 20:33

answered Jul 31, 2020 at 20:28

Zan Lynx

54.7k11 gold badges85 silver badges135 bronze badges

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