Values, Spaces, and Places
The result of an expression in Vine has one of three forms: a value, a space, or a place.
- a value represents some piece of data
- a space represents somewhere that data can be put
- a place is the combination of a value and a space.
We'll use the following code example to explore these concepts further.
let x = 1 + 2;
x + 1;
x = 5;
x *= 2;
On line 1, the expression 1 + 2 results in the value 3. The variable x is
then declared with an initial value of 3.
On line 2, the expression x results in its value 3, so the expression
x + 1 results in the value 4. This expression is unused so its result is
simply discarded.
On line 3, we have an assignment statement. This expects a space on the
left-hand side, and a value on the right-hand side, and will put the value into
the space. In this particular case, the expression x on the left-hand side
results in a space – whatever value is put into this space will become the new
value of x. Then, the assignment operator puts the value 5 into the space.
The value of x is now 5.
On line 4, the *= operator expects a place on the left-hand side, and a value
on the right-hand side. Thus, x results in its place, which is the combination
of the value 5 and a space to put a new value for x. The *= operator uses
the value to calculate 5 * 2, and puts the value 10 into x's space. Thus,
the value of x is now 10.
Expression Resolution
Expression Positions
Every expression is located in either a value position, a space position, or a place position. The form of the expression's position determines the form of the result of the expression.
We saw above that the expression x could result in a value, a space, or a
place; this is determined by the form of its position.
- On line 2, in
x + 1,xis in a value position, so it results in a value. - On line 3, in
x = 5,xis in a space position, so it results in a space. - On line 4, in
x *= 2,xis in a place position, so it results in a place.
Expression Forms
Independent of its position, every expression has a form - it is either a value
expression, a space expression, or a place expression. For example, variable
expressions, like x, are place expressions.
The form of an expression determines the form of what the expression evaluates to. If this form does not match the form of the expression's position, it is coerced to become the result.
- On line 2, in
x + 1,xevaluates to a place. It is in a value position, so it is coerced to a value. - On line 3, in
x = 5,xevaluates to a place. It is in a space position, so it is coerced to a space. - On line 4, in
x *= 2,xevaluates to a place. It is in a place position, so it is not coerced.
Coercion
There are two rules for coercion.
-
When a place is coerced to a value, the resulting value is a copy of the place's value; the other copy is put into the place's space.
-
When a place is coerced to a space, the resulting space is the place's space; the place's value is discarded.
These two rules result in the behavior described in the first section.
Values and spaces cannot be coerced. It is an error to have a value expression in a non-value position, or a space expression in a non-space position.