The design of MiniBasic

MiniBasic is designed to allow non-programmers to add arbitrary functions to programs. Imagine we are attempting to SPAM-shield an email program. Different users have different ideas of what constitutes SPAM. By providing checkboxes we can go so far, but if a user want, say to, regard as SPAM everything with an attachment, unless it is under a certain size, unless it has come from a trusted list of addresses, then we are stuck. However by providing a MiniBasic interface, the user can input the relevant values and provide the logic.

The calling program would do this by setting up the input stream with, say, the email address of the sender, the title of the email, the length of any attachment.

The calling program then presents half of a MiniBasic program to the user, with the input set up

e.g.

The user then provides the logic for his choice.

MiniBasic can of course also be used as a stand-alone console programming language. This is useful for teaching purposes, for testing MiniBasic programs, or if you simply want to write a filter program that accepts from standard input and writes to standard output.

It was important that MiniBasic be simple to learn, and simple to implement. For this reason the syntax of the language has been kept as close as possible to the type of BASIC used on microcomputers in the 1980s. Millions of people know a BASIC of this kind.

Because of advances in computer power since the 1980s array initialisation was allowed. This allows us to eliminate the difficult to use READ and DATA statements. Re-dimensioning of arrays was also allowed, largely for theoretical reasons (it turns MiniBasic into a Turing machine).

GOSUB was not included. It is not of much practical use without local variables and parameters, and a functional language isn't very useful without some mechanism for passing and returning vectors. Adding these would have complicated the design of the interpreter considerably, and moved the language away from original BASIC.

PEEK and POKE are obviously hardware-dependent, add potential security risks, and were also not included.

The C-language source to MiniBasic is included. The interface is

In the standalone program these are called with stdin, stdout, stderr. In a component environment, these will usually be temporary memory files, and the input will be set up with the parameters to the function the user is to write.

The function returns 0 on success or non-zero on failure.

The source code is portable ANSI C. With the exception of the CHR$() and ASCII() functions, which rely on the execution character set being ASCII. The relational operators for strings also call the function strcmp() internally, which may have implications on non-ASCII systems.

An interpreted language is obviously not a particularly efficient way of running functions. Variables are stored in linear lists, with an O(N) access time, so big programs are O(N*N). However because of the lack of support for subroutines MiniBasic is not very suitable for complex programs anyway. If you were to extend the scope of the program to run very large scripts, it would be necessary to replace the variable list with a hash table, binary tree, or other structure that supports fast searching. All MiniBasic keywords, with the exception of e, start with uppercase letters. This fact is exploited to allow faster recognition of identifiers starting with lower case. Users can use this feature to gain some performance advantage.

On a fast computer the efficiency of MiniBasic shouldn't be a major problem unless users run very processor-intensive scripts, or if the function is in a time-critical portion of code. In these cases the answer would be to move to a pseudo-compiler system, where the MiniBasic scripts are translated into an intermediate bytecode that is similar to machine language. This is a project for a later date.

Since MiniBasic is available as C source, it is possible to extend the language. Where possible extensions should be in the form of functions rather than new statements, to avoid changing the grammar of the language. To add a numerical function, foo, which takes a numerical and a string argument, write the function like this

foo - check the first character of a string FOO(85, "Useless function")

    
double foo(void)
{
  double answer;
  double x;
  char *str;
  int ch;

  match(FOO); /* match the token for the function */
  match(OPAREN); /*opening parenthesis */
  x = expr();    /* read the numerical argument */
  match(COMMA); /* comma separates arguments */
  str = stringexpr(); /* read the string argument */
  match(CPAREN); /* match close */
  if(str == NULL) /* computer can run out of memory */
  return 0;           /* stringexpr() will have signalled the error so no point in generating another */
  ch = integer(x); /* signal error if x isn't an integer */
  if( !isalpha(ch) )
    seterror(ERR_BADVALUE); /* signal an error of your if ch isn't  valid */

  if(str[0] == ch)         /* function logic */
    answer = 1.0;
  else
    answer = 2.0;

 free(str);               /* str is allocated with malloc(), so free */
 return answer;
}  

Once you have your function, add an identifier for it, FOO, to the token list. Then to the functions gettoken() and tokenlen() add the appropriate lines. Finally to the function factor() add the code for calling your function.

For string functions, the procedure is similar, except that they must return an allocated string. The convention is that they end with a dollar sign, and the token id ends with the sequence STRING. Add the call to the function stringexpr(), and add your symbol to the function isstring() so that statements like PRINT know that it generates a string expression.

To change the input and output model, you need only change the functions doprint() and doinput(). If you wish to change the error system then you need to look at the functions setup(), reporterror() and the top-level function basic(). Currently the program takes FILE pointers, which should be flexible enough for most uses, but not if say you want to provide for interactive scripts.