Implement a struct-to-csv writer in Go

Question

The following code attempt to implement a generic CSV writer for any simple struct. By "simple", I mean field value of the struct are of standard, simple types (int, string etc).

type (
    CSV interface {
        Header() []string
        String([]string) (string, error)
    }
    CSVArray []CSV
)


func CSVOutput(w io.Writer, data CSVArray, cols []string) error {
    if len(data) == 0 {
        return nil
    }
    _, err := fmt.Fprintln(w, data[0].Header())
    if err != nil {
        return err
    }
    for _, d := range data {
        str, err := d.String(cols)
        if err != nil {
            return err
        }
        _, err = fmt.Fprintln(w, str)
        if err != nil {
            return err
        }
    }
    return nil
}

The problem is CSVOutput() does not actually work. e.g.:

var data []Employee //the Employee struct implements CSV interface
CSVOutput(w, data, nil)

Compilation failed: cannot use data (type []Employee) as type CSVArray in argument to CSVOutput

I understand that []CSV is not same as []Employee, as explained here, and many other resources available online.

That said, is it possible to rewrite the CSVOutput() function by using reflection:

func CSVOutput(w io.Writer, data interfac{}, cols []string) error {
    sliceOfIntf = castToSlice(data) //how to do this?
    if !implementedCSV(sliceOfIntf[0]) { //and how to do this?
        return errors.New("not csv")
    }
    ... ...
}

pkg.go.dev/github.com/jszwec/csvutil#readme-marshal

rustyx
– rustyx

2021-11-25 15:52:05 +00:00
Commented Nov 25, 2021 at 15:52 — rustyx
– rustyx, Commented Nov 25, 2021 at 15:52

mkopriva · Accepted Answer · 2021-11-25 15:45:26Z

2

is it possible to rewrite the CSVOutput() function by using reflection

Yes

// if data is []Employee{...}, then you can do the following:

rv := reflect.ValueOf(data)
if rv.Kind() != reflect.Slice {
    return fmt.Errorf("data is not slice")
}
if !rv.Type().Elem().Implements(reflect.TypeOf((*CSV)(nil)).Elem()) {
    return fmt.Errorf("slice element does not implement CSV")
}

csvArr := make(CSVArray, rv.Len())
for i := 0; i < rv.Len(); i++ {
    csvArr[i] = rv.Index(i).Interface().(CSV)
}

// now csvArr is CSVArray containing all the elements of data

https://go.dev/play/p/gcSOid533gx

answered Nov 25, 2021 at 15:45

mkopriva

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3 Comments

xrfang Over a year ago

could you explain what is (*CSV)(nil)? Cast nil into a CSV interface? Can we make it CSV(nil) ?

mkopriva Over a year ago

"what is (*CSV)(nil)" -- that's an expression that converts nil to *CSV. "Can we make it CSV(nil)" -- Nope. You need pointer-to-interface because otherwise the interface-type information would be lost.

mkopriva Over a year ago

@xrfang for a more complete explanation see: stackoverflow.com/a/63938859/965900

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