Coercion

gopantic hands input to the standard decoder first. Only when that decoder rejects a value does coercion look at it, and it converts the value only if nothing is lost. Input that needs no conversion is decoded exactly as encoding/json or yaml.v3 would decode it, and one field needing conversion doesn't change how the others decode.

What converts

Into	From	Example
Integer kinds	Number strings, whole floats	"42" and 42.0 to 42
Floats	Number strings, integers	"3.14" to 3.14
bool	The strings below, numbers	"yes" to true, 0 to false
string	Numbers, booleans	42 to "42", true to "true"
time.Time	Date strings, Unix seconds	1704067200

The strings accepted as booleans are true, 1, yes, on and false, 0, no, off, in lower, title or upper case. The empty string is false.

Times parse as RFC 3339, 2006-01-02T15:04:05, 2006-01-02 15:04:05 or 2006-01-02. Unix seconds have to fall in years 1 to 9999.

The rules are the same inside slices, arrays, pointers and map values, and in map keys, so map[int]string reads {"1": "a"}.

What errors

Coercion refuses anything that would change the value:

Input	Target	Why
300	int8	Out of range
4.7	int	Fractional
-1	uint	Negative
1e60	float32	Out of range
1e16	int	Written as a float at or past 2^53, so it may have been rounded already
"NaN", "Inf"	Any float	Not a number gopantic will return
{"a": 1}	string	Would hand back Go syntax as if it were the input
1e300	time.Time	Past year 9999
abc	json.Number	Not a number

Whole numbers written out in full are read digit by digit, so 9007199254740993 stays exact in an int64. The rules are the same in YAML, where yaml.v3 alone would turn 1.9 into 1. A json.Number keeps the digits as written.

Types that decode themselves

Fields with an UnmarshalJSON, UnmarshalYAML or UnmarshalText method decode themselves, and json.RawMessage keeps its bytes as they arrived. Coercion leaves them alone, with one exception: time.Time falls back to the wider list of formats above.

Coercion cannot be turned off

If a string in a numeric field should be an error, gopantic is the wrong tool for that field. Decode with encoding/json, then call model.Validate:

var req Request
if err := json.Unmarshal(body, &req); err != nil {
    return err
}
err := model.Validate(&req)