# Pattern: Handler

## Responsibility

HTTP only: bind JSON/query → validate → call service interface → map errors to envelope.

## Shape

```go
type ReportService interface {
    Decode(ctx context.Context, in DecodeInput) (*DecodeOutput, error)
}

type ReportHandler struct {
    svc ReportService
}

func (h *ReportHandler) Decode(c *gin.Context) {
    var req DecodeRequest
    if err := c.ShouldBindJSON(&req); err != nil {
        response.Fail(c, 400, 10000, "invalid request")
        return
    }
    out, err := h.svc.Decode(c.Request.Context(), toInput(req))
    if err != nil {
        // map domain errors → codes
        response.Fail(c, 400, 30001, err.Error())
        return
    }
    response.OK(c, out)
}
```

## Never

- SQL here
- Business unlock math here (belongs service)