XO0404: type does not implement a required interface or constraint

error · reported by the checker

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

struct P { x: Int }

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

Fix

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.

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XO0404: type does not implement a required interface or constraint

Example

Fix