Keywords: pame

# Names

Previous chapters dealt with values that have always been known when the program was written. If a program is to be able to react to its environment, it must be able to convert external values into internal values and then manipulate them. Analogous to `print`, the conversion can be done by `read` which constructs internal values from external character sequences. In order to manipulate such converted values, we need some way of referring to them. Algol 68 can generate values which can refer to other values. This kind of value is called a name. Although a name has a value, it is quite different from the value referred to. The difference is rather like your name: your name refers to you, but is quite distinct from you.

For example, suppose `read` is presented with the character sequence “`123G`” and is expecting an integer. `read` will convert the digits into the number “one hundred and twenty-three”, held in a special internal form called “2's-complement binary”. To manipulate that value, a name must be generated to refer to it. The mode of a name is called a “reference mode”.

A name which can refer to a value of mode `INT` is said to have the mode `REF INT`. Likewise, we can create names with modes

       REF BOOL    REF[]CHAR    REF[,]REAL

As you can see, `REF` can precede any mode. It can also include a mode already containing `REF`. Thus it is possible to construct modes such as

       REF REF INT
       REF[]REF REAL
       REF[]REF[]CHAR
       REF REF REF BOOL

but we shall defer discussion of these latter modes to chapter 11.

Names are created using generators. There are two kinds of generator: local and global. The extent to which a name is valid is called its scope. The scope of a local name is restricted to the smallest enclosing clause which contains declarations. The scope of a global name extends to the whole program. In general, values have scope, identifiers have range. We shall meet global generators in chapters 6 and 11.

The phrase `LOC INT` generates a name of mode `REF INT` which can refer to a value of mode `INT`.6.1 The `LOC` stands for local. It is quite reasonable to write the phrase

       read(LOC INT)

Unfortunately, the created name is an anonymous name in the sense that it has no identifier so that once the `read` has completed, the name disappears. We need some way of linking an identifier with the generated name so that we can access the name after `read` has finished. This is done with an identity declaration. Here is an identity declaration with a local generator:

       REF INT a = LOC INT

The value identified by `a` has the mode `REF INT` because the phrase `LOC INT` generates a name of mode `REF INT`. Thus it is a name, and it can refer to a value (as yet undefined) of mode `INT` (the value referred to always has a mode of one less `REF`). So now, we can write

       read(a)

After that phrase has been elaborated, `a` identifies a name which now refers to an integer.

Names can also be declared using a predeclared name on the right-hand side of the identity declaration. Here is another identity declaration using `a`:

       REF INT b = a

In this declaration, `b` has the mode `REF INT` so it identifies a name. `a` also has the mode `REF INT` and therefore also identifies a name. The identity declaration makes `b` identify the same name as `a`. This means that if the name identified by `a` refers to a value, then the name identified by `b` (the same name) will always refer to the same value.

Subsections

- Assignment
  - Copying values
  - Assigning operators
  - Exercises
- Assignments in formulæ
  - Exercises
- Multiple names
- Assigning to multiple names
  - Individual assignment
  - Collective assignment
  - Exercises
- Flexible names
  - Exercises
- The mode `STRING`
  - Exercises
- Reference modes in transput
  - Exercises
- Dynamic names
  - Exercises
- Loops revisited
  - Exercises
- Abbreviated declarations
  - Exercises
- Summary
  - Exercises