go 55 lines · 7 steps

Parsing INI files in Go

A line-by-line scanner turns INI text into a nested map of sections and key-value pairs.

Explained by highlit
Intermediate parsing state-machine error-handling maps io
1package config
2 
3import (
4 "bufio"
5 "fmt"
6 "io"
7 "strings"
8)
9 
10type INI map[string]map[string]string
11 
12func ParseINI(r io.Reader) (INI, error) {
13 result := INI{}
14 section := ""
15 result[section] = map[string]string{}
16 
17 scanner := bufio.NewScanner(r)
18 for line := 1; scanner.Scan(); line++ {
19 text := strings.TrimSpace(scanner.Text())
20 if text == "" || strings.HasPrefix(text, ";") || strings.HasPrefix(text, "#") {
21 continue
22 }
23 
24 if strings.HasPrefix(text, "[") {
25 if !strings.HasSuffix(text, "]") {
26 return nil, fmt.Errorf("line %d: unterminated section header", line)
27 }
28 section = strings.TrimSpace(text[1 : len(text)-1])
29 if section == "" {
30 return nil, fmt.Errorf("line %d: empty section name", line)
31 }
32 if _, ok := result[section]; !ok {
33 result[section] = map[string]string{}
34 }
35 continue
36 }
37 
38 key, value, ok := strings.Cut(text, "=")
39 if !ok {
40 return nil, fmt.Errorf("line %d: expected key=value, got %q", line, text)
41 }
42 
43 key = strings.TrimSpace(key)
44 value = strings.Trim(strings.TrimSpace(value), `"'`)
45 if key == "" {
46 return nil, fmt.Errorf("line %d: empty key", line)
47 }
48 result[section][key] = value
49 }
50 
51 if err := scanner.Err(); err != nil {
52 return nil, fmt.Errorf("read config: %w", err)
53 }
54 return result, nil
55}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1A single mutable `section` variable acts as the parser's state, routing each key into the right map.
  2. 2Validating structure eagerly and returning errors with line numbers makes malformed input easy to diagnose.
  3. 3Seeding a default empty section lets keys appear before any header without special-casing.

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