10. Type Promotion
Type promotion is a sub-problem of casting and refers to casts that happen implicitly.
Any conversion that can be done implicitly via promotion can also be done explicitly via typecast expression. The notable exception is array promotion to a higher dimension, which occurs as a consequence of scalar to array promotion.
10.1. Scalars
The only automatic type promotion for scalars is integer to
real. This promotion is one way - a real cannot be automatically
converted to integer.
Automatic type conversion follows this table where N/A means no implicit
conversion possible, id means no conversion necessary,
as<toType>(var) means var of type “From type” is converted to type
“toType” using semantics from .
To type |
|||||
From type |
boolean |
character |
integer |
real |
|
boolean |
id |
N/A |
N/A |
N/A |
|
character |
N/A |
id |
N/A |
N/A |
|
integer |
N/A |
N/A |
id |
as<real>(var) |
|
real |
N/A |
N/A |
N/A |
id |
|
10.2. Scalar to Array
All scalar types can be promoted to arrays that have an internal type that the scalar can be converted to implicity. This can occur when an array is used in an operation with a scalar value.
The scalar will be implicitly converted to an array of equivalent dimensions and equivalent internal type. For example:
integer i = 1;
integer[*] v = [1, 2, 3, 4, 5];
integer[*] res = v + i;
res -> std_output;
would print the following:
[2 3 4 5 6]
Other examples:
1 == [1, 1] // True
1..2 || 3 // [1, 2, 3]
Note that an array can never be downcast to a scalar, even if type casting is used. Also note that matrix multiply imposes strict requirements on the dimensionality of the the operands. The consequence is that scalars can only be promoted to a matrix if the matrix multiply operand is a square matrix (\(m \times m\)).
10.3. Multi-dimensional Arrays
Array promotion to a higher dimension occurs because, for example, a row in a 2D array is equivalent to a 1D array. When a 2D array is initialized or operated on with a 1D array, each scalar element in the 1D array is interpreted as a row. By applying the scalar to array promotion rule, each scalar element in the array will be promoted to a row. The example below demonstrates scalar to row promotion, row padding and column padding all together.
integer[3][4] m1 = [1,[1,2,3]];
// m1 = [[1, 1, 1, 1], [1, 2, 3, 0], [0, 0, 0, 0]]
The number of columns in each row are inferred, first by the expression
indicating the column size in the type declaration, and second, if the columnn
size is inferred as *, by the size of the array literal or expression.
Therefore, when the column size is infered, an array to matrix promotion always
produces a square matrix.
integer[2][*] m2 = [3, 4];
// m2 = [[3, 3], [4, 4]]
Array promotions to a higher dimensionality apply in all contexts where operations on arrays are defined.
10.4. Tuple to Tuple
Tuples may be promoted to another tuple type if it has an equal number of internal types and the original internal types can be implicitly converted to the new internal types. For example:
tuple(integer, integer) int_tup = (1, 2);
tuple(real, real) real_tup = int_tup;
tuple(char, integer, boolean[2]) many_tup = ('a', 1, [true, false]);
tuple(char, real, boolean[2]) other_tup = many_tup;
Field names of tuples are overwritten by the field names of the left-hand side in assignments and declarations when promoted. For example:
tuple(integer a, real b) foo = (1, 2);
tuple(real c, real) bar = foo;
foo.a -> std_output; // 1
foo.b -> std_output; // 2
bar.a -> std_output; // error
bar.b -> std_output; // error
bar.c -> std_output; // 1
If initializing a variable with a tuple via Type Inference, the variable is assumed to be the same type. Therefore, field names are also copied over accordingly. For example:
tuple(real a, real b) foo = (1, 2);
tuple(real c, real d) bar = (3, 4);
var baz = foo;
baz.a -> std_output; // 1
baz.b -> std_output; // 2
baz = bar;
baz.a -> std_output; // 3
baz.b -> std_output; // 4
It is possible for a two sided promotion to occur with tuples. For example:
boolean b = (1.0, 2) == (2, 3.0);
10.5. Character Array to/from String
A string can be implicitly converted to an array of characters and vice-versa (two-way type promotion).
string str1 = "Hello"; /* str1 == "Hello" */
character[*] chars = str1; /* chars == ['H', 'e', 'l', 'l', 'o'] */
string str2 = chars || [' ', 'W', 'o', 'r', 'l', 'd']; /* str2 == "Hello World" */