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Yasuhiro Matsumoto
Yasuhiro Matsumoto

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Go's method is curried funtion

#go

Go's method is not same in other languages. In C++, this should not be null.

class Foo { public: void doSomething() {} }; int main() { Foo *foo = nullptr; foo->doSomething(); }
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Go's method does not require that the receiver is instanciated.

package main import ( "fmt" ) type Foo struct { } func (f *Foo) doSomething() { fmt.Println(f) } func main() { var foo *Foo foo.doSomething() }
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And the method hold receiver value. This is called Method Values.

package main import ( "fmt" ) type Foo struct { Value string } func (f *Foo) WhatsUp() { fmt.Println(f.Value) } func doSomething(f func()) { f() } func main() { foo := &Foo{Value: "Hello, World"} doSomething(foo.WhatsUp) }
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The WhatsUp is evaulated with the receiver.

package main import ( "fmt" ) type Foo int func (f Foo) doSomething() { fmt.Println(f) } func main() { Foo.doSomething(123) }
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So this code is valid. Foo.doSomething is the function object. It can be called with let the function object to a variable. The function type should take an first argument which is type of the receiver.

package main import ( "fmt" ) type Foo int func (f Foo) doSomething() { fmt.Println(f) } func main() { var fn func(Foo) fn = Foo.doSomething fn(123) }
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This idiom is useful to call runtime.SetFinalizer to Close something resource must be closed.

package main import ( "log" "os" "runtime" "myresource" ) func main() { f := myresource.OpenSomethingMustBeClosed() if err != nil { log.Fatal(err) } runtime.SetFinalizer(f, (*myresource.Writer).Close) }
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Top comments (4)

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nekketsuuu profile image
Takuma Ishikawa •

Interesting! However, what do you mean by the words "curried function" in the title? I think Foo.doSomething is not curried. For example, if doSomething takes one argument, Foo.doSomething(123) causes an error.

func (f Foo) doSomething(b bool) { fmt.Println(f, b) } func main() { Foo.doSomething(123) // error: not enough arguments }

Maybe you want to say that Go can pass a receiver to a method value as an argument?

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mattn profile image
Yasuhiro Matsumoto • • Edited

Yes, in strictly, as you think, this is not currying. The currrying is a transform taking one less argument.

package main import ( "fmt" ) func add(n int) func(int) int { return func(v int) int { return n + v } } func main() { add10 := add(10) fmt.Println(add10(5)) }

Just metaphor :)

If this function object for currying, you can try this.

package main import ( "fmt" ) type number int func (f number) add(b number) number { return f + b } func main() { fmt.Println(number(3).add(4)) fmt.Println(number.add(3, 4)) }
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der_gopher profile image
Alex Pliutau •

I never saw Go people use Foo.doSomething(123) ... Can you point to some open source projects doing this?

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mattn profile image
Yasuhiro Matsumoto •

Yes, most of cases, the function will not be called directly. As I wrote in above, it is often used as function object. See os/exec.go in Go repository.

runtime.SetFinalizer(p, (*Process).Release)