Ok, I've been programming in Go for a few days so I could be considered a newbie, but I cannot figure out how to import the definition of the File struct. I want to pass a *File into a func and I can't seem to get File defined. I'm importing "os" and calling os.Create in my main. How do I import the right item or declare the parameter in my func definition to pass a file pointer?
import "os"
func testfunc(fp *File) { ... }
fp := os.Create("myfile")
testfunc(fp)
Your testfunc declaration should look like this:
func testfunc(fp *os.File) { ... }
I was stuck about half an hour on this one.
Anyway, I hope this helps some other people as well.
Give the qualified path to the struc File
import "os"
func testfunc(fp *os.File) { ... }
fp := os.Create("myfile")
testfunc(fp)
Related
I could override the builtin print() function's behavior by defining another print() in scope, as in https://play.golang.org/p/Y2ly31oXU67
Is it possible in go to alter the behavior on-the-fly of an imported function, say fmt.Println()?
If you want to 'alter' a builtin function, look at the very fine monkey patch utility https://github.com/bouk/monkey (And pay attention to the warnings, it's only really useful in test functions, and I for one reject any prod code that imports that package)
Import a different package with name "fmt" and implement whatever functions you need in that package. Here's an example:
File go.mod:
module test
File main.go
package main
import (
"test/fmt"
)
func main() {
fmt.Println("Hello, playground")
}
File fmt/fmt.go:
package fmt
import (
"fmt"
"log"
)
func Println(format string, args ...interface{}) {
msg := fmt.Sprintf(format, args...)
log.Printf(msg)
}
Run it on the playground.
The code in this answer does not modify the imported function as asked in the question.
I'm discovering generator (go generate) and I'm trying to generate Validation function for my struct.
The idea is that I don't want my program to use reflect at runtime, I would rather have a generator use reflect to generate the actual method I want to use.
the problem is I can't import my structs in the generator code, the only way I found so far was to read the .go file from the generator and manually parse the types defined there using regex
I've got something like
models/models.go:
package models
//go:generate go run ../generator.go -file models.go
type MyStruct struct {
...
}
generator.go:
package main
func main() {
f, err := ioutil.ReadFile(fileName) // I read filename from the flag provided
...
// I parse f to generate my stuff
}
I would very much prefer to have an introspection package that would take a go code as a string and give me some information about the struct defined there
Or maybe there is a way to import the file that call go:generate to get directly access to the types
There is no need to specify file name , this code does the same :
//go:generate go run ../generator.go -file $GOFILE
With help of text/template package you are needless of parsing the file. A very simple example would be something like this. This will give you the clue :
package main
import (
"flag"
"os"
"text/template"
)
//go:generate go run main.go -name=A
//go:generate go run main.go -name=B
//go:generate go run main.go -name=C
var name = flag.String("name", "test", "name of struct")
var code = `
package main
type Struct{{.}} struct {}
func (s *Struct{{.}} ) Vailadte() bool {
return true
}
`
func main() {
flag.Parse()
file, _ := os.Create(*name + ".go")
defer file.Close()
tmpl, _ := template.New("test").Parse(code)
tmpl.Execute(file, *name)
}
Maybe you can utilize go/parser and go/ast in your generator.
I am new to go programming language and I'm stock on this scenario on my code.
Here's my example code:
a := genreAPI{Genre{"Pop"}, Genre{"Rock"}}
fmt.Println("Value of a :", a)
The current output is: Value of a : [{Pop} {Rock}]
How can I achieved an output like this:
Value of a : [{UG9w} {Um9jaw==}]
which is a base64 encode?
I am not sure what exactly is not clear from the documentation. Not only it has a clear name which explains states what the method is doing, it also has an example.
package main
import (
"encoding/base64"
"fmt"
)
func main() {
data := []byte("Pop")
str := base64.StdEncoding.EncodeToString(data)
fmt.Println(str) // UG9w
}
Go Playground
You can customise the output of print functions by providing a String() method for your type. Either for the whole Genre or just for the name variable.
Example:
package main
import (
"encoding/base64"
"fmt"
)
type Base64String string
func (b Base64String) String() string {
return base64.StdEncoding.EncodeToString([]byte(b))
}
type Genre struct {
Name Base64String
}
func main() {
a := []Genre{Genre{"Pop"}, Genre{"Rock"}}
fmt.Println(a) // prints [{UG9w} {Um9jaw==}]
fmt.Println(string(a[0].Name)) // prints Pop
}
I've expected tag to be "account" but it is "ccount". Why is "a" removed?
package main
import "fmt"
import "strings"
func main() {
s := "refs/tags/account"
tag := strings.TrimLeft(s, "refs/tags")
fmt.Println(tag)
}
Run
Use TrimPrefix instead of TrimLeft
package main
import "fmt"
import "strings"
func main() {
s := "refs/tags/account"
tag := strings.TrimPrefix(s, "refs/tags/")
fmt.Println(tag)
}
Please notice that following TrimLeft calls will result the same "fghijk
" string:
package main
import (
"fmt"
"strings"
)
func main() {
s := "/abcde/fghijk"
tag := strings.TrimLeft(s, "/abcde")
fmt.Println(tag)
tag = strings.TrimLeft(s, "/edcba")
fmt.Println(tag)
}
So TrimLeft is not the method which fits your needs. I guess it's impossible to use it in the example you've given to get the result you expect.
It is working as documented:
TrimLeft returns a slice of the string s with all leading Unicode
code points contained in cutset removed
Because there's an 'a' in the first argument (the cutset) the leading 'a' in account is removed
I'm trying to write an application to pull status from a database, but I seem to be getting stuck on a really basic principle of the language. I have the program written, but it doesn't compile due to the error use of package time not in selector.
A really basic example (from play.golang.org's own test environment)
package main
import (
"fmt"
"time"
)
func main() {
s_str := time.Now()
fmt.Println( printT(s_str) )
}
func printT(t time) time {
return t.Add(100)
}
Unfortunately, I've found documentation and helpdocs online a bit wanting. My understanding is that the import statement should include the library for the entire program like in C++ correct?
You have to prefix the imported types or variables with the name you gave to the package in the import (here you use the default name, that is "time"). That's what you did for the function Now but you have to do it also for the types.
So the type isn't time but time.Time (that is : the type Time that is declared in the package you import with the name "time").
Change your function to
func printT(t time.Time) time.Time {
return t.Add(100)
}
And for your second question : No, the import statement doesn't include the library for the entire program but only for the current file.