struct ExpressionEmitter{
    method buildI8(self:ref ExpressionEmitter, val:i8):gc[Expression];
    method buildI16(self:ref ExpressionEmitter, val:i16):gc[Expression];
    method buildI32(self:ref ExpressionEmitter, val:i32):gc[Expression];
    method buildI64(self:ref ExpressionEmitter, val:i64):gc[Expression];
    method buildString(self:ref ExpressionEmitter, val:ptr[] u8):gc[Expression];
    method buildCall(self:ref ExpressionEmitter, head:gc[Expression], args:slice[gc[Expression]]):gc[Expression];
    ....
}


class ToExpression[T]{
    method materialize(emitter:ref ExpressionEmitter, val:ref T):gc[Expression];
}

instance ToExpression[i32]{
    method materialize(emitter:ref ExpressionEmitter, val:ref i32):gc[Expression]{
        return emitter.buildI32(*val);
    }
}

...

struct MyData{
    field x y:i32;
}

instance ToExpression[MyData]{
    method materialize(emitter:ref ExpressionEmitter, val:ref MyData):gc[Expression]{
        let constr = emitter.buildConstructor(reflect(MyData));
        constr.addField(reflect(MyData.x), @val->x);
        constr.addField(reflect(MyData.y), @val->y);
        return constr.toExpression();
    }
}