比较命名类型和未命名类型时发生编译错误

Question

package main

import (
    "fmt"
    "html/template"
)

func main() {
    fmt.Println(template.HTML("test") == "test")

    htmlString := "test"
    fmt.Println(template.HTML("test") == htmlString)
}

http://play.golang.org/p/dON4eLpGN8

document for template.HTML: http://golang.org/pkg/html/template/#HTML

The first comparison is true. However, the second comparison yield following error :

invalid operation: "html/template".HTML("test") == htmlString (mismatched types "html/template".HTML and string)

Can someone explain what happened under the hood ?

Answer 1

In short, second expression is invalid their types are incompatible.

Every operations in Go, arguments are must be same type. The second expression

template.HTML("test") == htmlString

is invalid since it's comparing template.HTML and string. Although template.HTML is driven from string, but it is incompatible. You should cast the variable like template.HTML(htmlString).

But the first expression

template.HTML("test") == "test"

is valid because the type of constant "test" interpreted as template.HTML. Untyped constant has default type, but it can be any driven type by context at compile time. This article explains about constant in detail. Maybe this article make your question clear.