# XO0404: type does not implement a required interface or constraint

The value is used in a way that needs an interface its type does not implement: interpolation needs `Display`, `==` needs `Eq`, `<` needs `Ord`, `json.encode` needs `Json`, a generic call needs its constraint (`T: Ord`), and `derive X` needs every field to implement `X`. A value that holds a handle (`Mutex`, `Atomic`, `Chan`, `Task`, `Conn`, ...) or a function value is never `Eq` or `Hash`: it cannot be compared, be a Map key or Set element, or derive `Eq`, `Hash`, or `Ord` (decision 0112); compare an identifying field instead. Add `derive` to the type, implement the interface's methods, or convert the value first. See core 2.4 and 2.9.

## Example

```xo
struct P { x: Int }

fn f(p: P) -> Str { "${p}" }
```

## Fix

```xo
struct P { x: Int } derive Display

fn f(p: P) -> Str { "${p}" }
```

Run `xo explain XO0404` for this text in a terminal, or see
[section 10.6 of the specification](../../spec/core/#106-diagnostic-codes).