Get the type name of a generic struct without type parameters-CodePudding

Say I have a generic struct called foo and I create two objects from it. I can determine the concrete type of each using reflect.TypeOf(), like so:

package main

import (
    "fmt"
    "reflect"
)

type foo[T any] struct {
    data T
}

func main() {
    a := foo[string]{"cheese"}
    b := foo[int]{42}

    fmt.Println(reflect.TypeOf(a))
    fmt.Println(reflect.TypeOf(b))
}

// main.foo[string]
// main.foo[int]

What I am interested in is determining just the generic type of these objects (i.e., foo) and not the concrete type (i.e., foo[string] and foo[int]). Is this possible or do I need to manually extract the generic type from these strings (e.g., with regex)?

Edit

Regex might look something like this:

func GetGenericType(x any) string {
    // Get type as a string
    s := reflect.TypeOf(x).String()

    // Regex to run
    r := regexp.MustCompile(`\.(.*)\[`)

    // Return capture
    return r.FindStringSubmatch(s)[1]
}


fmt.Println(GetGenericType(a))
fmt.Println(GetGenericType(b))

// foo
// foo

I've also seen this question but this doesn't answer this question because it gives the concrete type (i.e., main.foo[string]) rather than the generic type (i.e., foo).

CodePudding user response：

Reflection doesn't see the name of the "base" generic type, because at run time that base type doesn't exist.

The relevant passage from the Go spec is Instantiations:

Instantiating a type results in a new non-generic named type; instantiating a function produces a new non-generic function.

So when you write:

b := foo[int]{42}
name := reflect.TypeOf(b).Name()

the name of that type is precisely foo[int].

It's worth noting that the identifier foo without the type parameter list is relevant at compile time, because it prevents you from redeclaring it in the same package. Type definitions:

A type definition creates a new, distinct type with the same underlying type and operations as the given type and binds an identifier, the type name, to it.
TypeDef = identifier [ TypeParameters ] Type .

But instantiations, as defined above, result in a new named type which is different than foo; and at run time when you can use reflection, you deal with instantiations only.

In conclusion, I think your solution with regex is acceptable, until some helper function is added to the stdlib (if ever).

Just keep in mind the difference between Type.String() and Type.Name(): any type can have a string representation, but only named types have a name. (Obviously, right?). So for example if you wrote:

b := &foo[int]{42}

then the type of b is *foo[int], which is an anonymous composite type, and Name() returns an empty string.