Variable Declaration
Synopsis
Declare a variable.
Syntax
Type Name = Exp ;Type Name;
Types
Type | Exp |
|---|---|
Type | <: Type |
Description
The effect of a variable declaration is to introduce a new variable Name and to assign the value of expression Exp to Name. A mention of Name later on in the same scope will be replaced by this value, provided that Name\'s value has not been changed by an intermediate assignment.
When a variable is declared, it has as scope the nearest enclosing block, or the module when declared at the module level.
The following rules apply:
Double declarations in the same scope are not allowed.
The type of Exp should be compatible with Type, i.e., it should be a subtype of Type.
As a convenience, also declarations without an initialization expression are permitted inside functions (but not at the module level) and have the form
_Type_ _Name_;
and only introduce the variable Name.
Rascal provides local type inference, which allows the implicit declaration of variables that are used locally in functions. The following rules apply:
An implicitly declared variable is declared at the level of the current scope, this may the whole function body or a block nested in it.
An implicitly declared variable gets as type the type of the first value that is assignment to it.
If a variable is implicitly declared in different execution path of a function, all these implicit declarations should result in the same type.
All uses of an implicitly declared variable must be compatible with its implicit type.
Examples
Two explicit variable declarations:
rascal>int max = 100;
int: 100
rascal>min = 0;
int: 0
An implicit variable declaration
rascal>day = {<"mon", 1>, <"tue", 2>, <"wed",3>,
>>>>>>> <"thu", 4>, <"fri", 5>, <"sat",6>, <"sun",7>};
rel[str,int]: {
<"thu",4>,
<"tue",2>,
<"sat",6>,
<"wed",3>,
<"fri",5>,
<"sun",7>,
<"mon",1>
}
Variable declaration and assignment leading to type error
rascal>int month = 12;
int: 12
rascal>month ="December";
|prompt:///|(7,10,<1,7>,<1,17>): Expected int, but got str
Advice: |https://www.rascal-mpl.org/docs/Rascal/Errors/CompileTimeErrors/UnexpectedType|
ok
Pitfalls
- Local type inference for variables always uses the smallest possible scope for a variable; this implies that a variable introduced in an inner scope is not available outside that scope. Here is how things can go wrong:
rascal>if( 4 > 3){ x = "abc"; } else { x = "def";}
str: "abc"
---
abc
---
rascal>x;
|prompt:///|(0,1,<1,0>,<1,1>): Undeclared variable: x
Advice: |https://www.rascal-mpl.org/docs/Rascal/Errors/CompileTimeErrors/UndeclaredVariable|
ok