Parsing with strconv

Everything that arrives from outside a program - a flag, an environment variable, a form field, a line of CSV - arrives as a string. Turning it into a number or a boolean is the strconv package's job, and every function in it returns two values: the parsed result and an error.

n, err := strconv.Atoi("42")             // 42, nil
f, err := strconv.ParseFloat("1.5", 64)  // 1.5, nil
b, err := strconv.ParseBool("true")      // true, nil

Atoi is shorthand for ParseInt(s, 10, 0). Going the other way is strconv.Itoa(n), FormatFloat, FormatBool.

Two things catch people out. First, these functions are strict: no surrounding whitespace, no trailing junk. strconv.Atoi(" 42 ") is an error, so a strings.TrimSpace usually comes first. Second, the error is not a bare message. It is a *strconv.NumError holding the function name, the input, and one of two sentinel errors:

_, err := strconv.Atoi("abc")
errors.Is(err, strconv.ErrSyntax)   // true - it was not a number at all
_, err = strconv.Atoi("999999999999999999999999")
errors.Is(err, strconv.ErrRange)    // true - it was a number, but too big

That is why a parse error is worth passing along unchanged: rewrite it with your own fmt.Errorf message and the caller can no longer tell "not a number" from "too large".

The other habit worth building is the Or wrapper. Configuration usually has a default, and

func ParseIntOr(s string, fallback int) int

turns a two-value call into a one-value one at the single place that knows what the default is.

Task

Fill in the six functions in main.go.

1. ParseIntOr, ParseFloatOr and ParseBoolOr parse a trimmed s and fall back when it does not parse.
2. SumFields(csv) adds up comma-separated integers, skipping empty fields, and on the first bad field returns 0 and strconv's own error.
3. ParseKeyValues(s) reads "a=1,b=2" into a map[string]int.
4. JoinInts(nums) renders a slice back into "1,2,3".

Hints

• strings.TrimSpace before every parse, not after.
• Return the error from strconv directly. The test uses errors.Is(err, strconv.ErrSyntax) and errors.Is(err, strconv.ErrRange), so wrapping it in a message of your own with %v would break the link.
• strings.Cut(entry, "=") returns the part before, the part after, and whether the separator was there at all - a better fit than strings.Split when there is exactly one separator to find.
• ParseKeyValues must return a non-nil map for an input with no entries, and nil alongside an error.
• strconv.Itoa is the fast path for int. fmt.Sprintf("%d", n) gives the same string and does much more work to get there.