### Declaration Basics

The s d (`calc-declare-variable`) command is the easiest way to make a declaration for a variable. This command prompts for the variable name, then prompts for the declaration. The default at the declaration prompt is the previous declaration, if any. You can edit this declaration, or press C-k to erase it and type a new declaration. (Or, erase it and press RET to clear the declaration, effectively "undeclaring" the variable.)

A declaration is in general a vector of type symbols and range values. If there is only one type symbol or range value, you can write it directly rather than enclosing it in a vector. For example, s d foo RET real RET declares `foo` to be a real number, and s d bar RET [int, const, [1..6]] RET declares `bar` to be a constant integer between 1 and 6. (Actually, you can omit the outermost brackets and Calc will provide them for you: s d bar RET int, const, [1..6] RET.)

Declarations in Calc are kept in a special variable called `Decls`. This variable encodes the set of all outstanding declarations in the form of a matrix. Each row has two elements: A variable or vector of variables declared by that row, and the declaration specifier as described above. You can use the s D command to edit this variable if you wish to see all the declarations at once. See section Other Operations on Variables, for a description of this command and the s p command that allows you to save your declarations permanently if you wish.

Items being declared can also be function calls. The arguments in the call are ignored; the effect is to say that this function returns values of the declared type for any valid arguments. The s d command declares only variables, so if you wish to make a function declaration you will have to edit the `Decls` matrix yourself.

For example, the declaration matrix

```[ [ foo,       real       ]
[ [j, k, n], int        ]
[ f(1,2,3),  [0 .. inf) ] ]
```

declares that `foo` represents a real number, `j`, `k` and `n` represent integers, and the function `f` always returns a real number in the interval shown.

If there is a declaration for the variable `All`, then that declaration applies to all variables that are not otherwise declared. It does not apply to function names. For example, using the row `[All, real]' says that all your variables are real unless they are explicitly declared without `real` in some other row. The s d command declares `All` if you give a blank response to the variable-name prompt.