json.Unmarshal vs. json.NewDecoder

README.md

Go json unmarshal vs using the decoder

When running main_unmarshal.go:

$ go run ./main_unmarshal.go
Input 2:
json syntax error: unexpected end of JSON input at offset 144

When running main_decoder.go:

$ go run ./main_decoder.go
Input 2:
json default: unexpected EOF%

Why?

input2.json

[
  {
    "id": "d421e09c-3479-11e7-90b6-7831c1ca8e1e",
    "created_at": "2017-05-09T10:00:00Z",
    "numbers": [
      12,
      43,
      54

main_decoder.go

package main

import (
  "encoding/json"
  "fmt"
  "os"
  "path/filepath"
  "time"
)

type link struct {
  Href string `json:"href"`
}

type links struct {
  Self   link `json:"self"`
  Parent link `json:"parent"`
}

type numberItem struct {
  ID        string    `json:"id"`
  CreatedAt time.Time `json:"created_at"`
  Numbers   []int     `json:"numbers"`
  Links     links
}

func readFile(filename string) {
  f, err := os.Open(filename)
  if err != nil {
    fmt.Printf("readFile: %s", err)
    os.Exit(0)
  }

  ni := make([]numberItem, 0)
  err = json.NewDecoder(f).Decode(&ni)
  if err != nil {
    switch err := err.(type) {
    case *json.SyntaxError:
      fmt.Printf("json syntax error: %s at offset %d\n", err, err.Offset)
    default:
      fmt.Printf("json default: %s", err)
    }
    os.Exit(0)
  }
}

func main() {
  fmt.Println("Input 2:")
  readFile(filepath.Join(".", "input2.json"))
}

main_unmarshal.go

package main

import (
  "encoding/json"
  "fmt"
  "io/ioutil"
  "os"
  "path/filepath"
  "time"
)

type link struct {
  Href string `json:"href"`
}

type links struct {
  Self   link `json:"self"`
  Parent link `json:"parent"`
}

type numberItem struct {
  ID        string    `json:"id"`
  CreatedAt time.Time `json:"created_at"`
  Numbers   []int     `json:"numbers"`
  Links     links
}

func readFile(filename string) {
  data, err := ioutil.ReadFile(filename)
  if err != nil {
    fmt.Printf("readFile: %s", err)
    os.Exit(0)
  }

  ni := make([]numberItem, 0)
  err = json.Unmarshal(data, &ni)
  if err != nil {
    switch err := err.(type) {
    case *json.SyntaxError:
      fmt.Printf("json syntax error: %s at offset %d\n", err, err.Offset)
    default:
      fmt.Printf("json default: %s", err)
    }
    os.Exit(0)
  }
}

func main() {
  fmt.Println("Input 2:")
  readFile(filepath.Join(".", "input2.json"))
}

Got a nice reply from @ivanjovanovic:

https://twitter.com/ivanjovanovic/status/869217714153324544

First takes file handle and expects to read more from it. Second takes data and says it has invalid syntax.