type User struct {
    ent.Schema
}

func (User) Hooks() []ent.Hook {
    return []ent.Hook{
        func(next ent.Mutator) ent.Mutator {
            return ent.MutateFunc(func(ctx context.Context, m ent.Mutation) (ent.Value, error) {
                // m's concrete type is: *ent.UserMutation. 
                return next.Mutate(ctx, m)
            })
        },
        // More hooks.
        // ...
    }
}

client.Use(func(next ent.Mutator) ent.Mutator {
    return ent.MutateFunc(func(ctx context.Context, m ent.Mutation) (ent.Value, error) {
        fmt.Println("mutation start")
        defer fmt.Println("mutation end")
        return next.Mutate(ctx, m)
    })
})

client.User.Use(func(next ent.Mutator) ent.Mutator {
    return ent.MutateFunc(func(ctx context.Context, m ent.Mutation) (ent.Value, error) {
        fmt.Println("user mutation start")
        defer fmt.Println("user mutation end")
        return next.Mutate(ctx, m)
    })
})

type Value interface{}

type Mutation interface {
    // Op returns the operation name generated by entc.
    Op() string
    // Type returns the schema type for this mutation.
    Type() string

    // Fields returns all fields that were changed during
    // this mutation. Note that, in order to get all numeric
    // fields that were in/decremented, call AddedFields().
    Fields() []string
    // Field returns the value of a field with the given name.
    // The second boolean value indicates that this field was
    // not set, or was not define in the schema.
    Field(name string) (Value, bool)
    // SetField sets the value for the given name. It returns an
    // error if the field is not defined in the schema, or if the
    // type mismatch the field type.
    SetField(name string, value Value) error

    // AddedFields returns all numeric fields that were incremented
    // or decremented during this mutation.
    AddedFields() []string
    // AddedField returns the numeric value that was in/decremented
    // from a field with the given name. The second value indicates
    // that this field was not set, or was not define in the schema.
    AddedField(name string) (Value, bool)
    // AddField adds the value for the given name. It returns an
    // error if the field is not defined in the schema, or if the
    // type mismatch the field type.
    AddField(name string, value Value) error

    // ClearedFields returns all nullable fields that were cleared
    // during this mutation.
    ClearedFields() []string
    // FieldCleared returns a boolean indicates if this field was
    // cleared in this mutation.
    FieldCleared(name string) bool
    // ClearField clears the value for the given name. It returns an
    // error if the field is not defined in the schema.
    ClearField(name string) error

    // ResetField resets all changes in the mutation regarding the
    // given field name. It returns an error if the field is not
    // defined in the schema.
    ResetField(name string) error

    // AddedEdges returns all edge names that were set/added in this
    // mutation.
    AddedEdges() []string
    // AddedIDs returns all ids (to other nodes) that were added for
    // the given edge name.
    AddedIDs(name string) []Value

    // RemovedEdges returns all edge names that were removed in this
    // mutation.
    RemovedEdges() []string
    // RemovedIDs returns all ids (to other nodes) that were removed for
    // the given edge name.
    RemovedIDs(name string) []Value

    // ClearedEdges returns all edge names that were cleared in this
    // mutation.
    ClearedEdges() []string
    // EdgeCleared returns a boolean indicates if this edge was
    // cleared in this mutation.
    EdgeCleared(name string) bool
    // ClearEdge clears the value for the given name. It returns an
    // error if the edge name is not defined in the schema.
    ClearEdge(name string) error

    // ResetEdge resets all changes in the mutation regarding the
    // given edge name. It returns an error if the edge is not
    // defined in the schema.
    ResetEdge(name string) error
}

func M(ctx context.Context, m ent.Mutation) (ent.Value, error) {
    v, ok := m.Field(user.FieldName) // or, m.Field("name")
    if ok {
        // ...
    }
    // An error is returned when field does not exist in the
    // schema, or the value type mismatch the field type.
    if err := m.SetField(user.FieldName, "boring"); err != nil {
        return nil, err
    }
    // ...
}

func M(ctx context.Context, m ent.Mutation) (ent.Value, error) {
    type NameSetter interface {
        Name() (string, bool)
        SetName(string)
    }
    ns, ok := m.(NameSetter)
    if !ok {
        // ignore.
    }
    name, ok := ns.Name()
    if ok {
         // ...
    }
    ns.SetName("boring")
    // ...
}

client.Card.Use(func(next ent.Mutator) ent.Mutator {
    return hook.UserFunc(func(ctx context.Context, m *ent.UserMutation) (ent.Value, error) {
        // Do something.
        return next.Mutate(ctx, m)
    })
})