package main

import (
    "encoding/json"
    "io/ioutil"
    "log"
    "os"
)

func main() {
    if len(os.Args) != 2 {
        log.Fatal("Usage: jsonformat.go <filename>")
    }

    byt, err := ioutil.ReadFile(os.Args[1])
    if err != nil {
        log.Fatalf("ERROR: Unable to read file %q: %v\n", os.Args[1], err)
    }

    var dat map[string]interface{}

    if err := json.Unmarshal(byt, &dat); err != nil { // this err is not flagged as masking the err in the outer scope, incorrectly
        log.Fatalf("ERROR: Invalid JSON file  '%v': %v\n", os.Args[1], err)
    }

    if output, err := json.MarshalIndent(dat, "", "  "); err != nil { // this err is flagged as masking the err in the outer scope, correctly
        log.Fatalf("ERROR: Unable to indent JSON file: %v\n", os.Args[1])
    } else {
        os.Stdout.Write(append(output, '\n'))
    }
}