How to update default values from http.Request elegantly? - go

Question

I'm doing book gopl's exercise 1.12, Basically, the code need to update several default values from http.Request if it is present in URL parameters.
Say here is the code I'm working on:
var ( // Need to update those values if corresponding parameter present in URL
cycles = 5
res = 0.001
size = 100
)
I can do the updating one by one:
if c := r.FormValue("cycles"); c != "" { // r is a *http.Request
i, err := strconv.ParseInt(c); err != nil {
cycles = i
}
}
if r := r.FormValue("res"); r != "" {
if f, err := strconv.ParseFloat(r); err != nil {
res = f
}
}
// ...
But I'm not satisfied by this solution:
If I have dozens of params, this is very cumbersome
How to handle the conversion errors?
The repeating pattern seems requires a function, like this (I don't know how to implement it yet, just showing my thought)
func setParam(p interface{}, name string, r *http.Request) error {
if f := r.FormValue(name); f != "" {
switch p.(type) {
case int:
// strconv.ParseInt
case float64:
// strconv.PraseFloat
// ...
}
}
This looks better, but still cumbersome. I don't know if this is the best solution. Or I overlooked some feature in Go that should be used in this situation.
So, what's the idiomatic way to do this?

Answer 1

Write functions that get a form value as a specific type or return default when value is missing. Example:
func intValue(r *http.Request, name string, def int) (int, error) {
if _, ok := r.Form[name]; !ok {
return def, nil
}
return strconv.Atoi(r.FormValue(name))
}
Call these functions from your handler. This is repetitive like the code in the question, but combines variable declaration, default value and fetching value in a single line of code.
cycles, err := intValue(r, "cycles", 5)
if err != nil {
// TODO; handle bad value
}