Default Values
Default values do not work yet.
While default values are able to be parsed in the data definition itself, the constructor of data cannot be called with the _ operator to create a default instance of the type.
Sometimes, you may want a field to have a default value. In Flint, this is done by assigning a value to the field directly in its declaration.
data MyData:
i32 x = 5;
i32 y;
MyData(x, y);
When instantiating this data module, you can use _ to signify using the default value for a field. The _ operator is only used in unused or default contexts, nowhere else. So, if you see a single _ in Flint you can always assume that either something is unused or set to a default.
use Core.print
data MyData:
i32 x = 5;
i32 y;
MyData(x, y);
def main():
MyData d = MyData(_, 20); // x uses the default value of 5
print($"d.x = {d.x}, d.y = {d.y}\n");
This program will print this line to the console:
d.(x, y) = (5, 20)
If all fields of a given data type have default values set the constructor of the data type can be called with a single _ operator to singify to set every field to its default value. But, note that this only works if every field in the given data type has a default value set. If one of them has no default value set this will fail.
use Core.print
data MyData:
i32 x = 5;
i32 y = 7;
MyData(x, y);
def main():
MyData d = MyData(_);
print($"d.(x, y) = ({d.x}, {d.y})\n");
This program will print this line to the console:
d.(x, y) = (5, 7)
- Default values simplify initialization but are optional.
- If a field doesn’t have a default value, using
_will result in a compiler error