Introduction

This is tinyscript, a scripting language designed for very tiny machines. The initial target is boards using the Parallax Propeller, which has 32KB of RAM, but the code is written in ANSI C so it should work on any platform (e.g. testing is done on x86-64 Linux).

On the propeller, the interpreter code needs about 3K of memory in CMM mode or 5K in LMM. On the x86-64 the interpreter code is 6K. The size of the workspace you give to the interpreter is up to you, although in practice it would be not very useful to use less than 2K of RAM. The processor stack is used as well, so it will need some space.

tinyscript is copyright 2016 Total Spectrum Software Inc. and released under the MIT license. See the COPYING file for details.

The Language

The scripting language itself is pretty minimalistic. The grammar for it looks like:

<program> ::= <stmt> | <stmt><sep><program>
<stmt> ::= <vardecl> | <funcdecl> |
           | <assignment> | <ifstmt>
           | <whilestmt> | <funccall>
           | <printstmt> | <returnstmt>

The statements in a program are separated by newlines or ';'.

Either variables or functions may be declared.

<funcdecl> ::= "func" <symbol> "(" <varlist> ")" <string>
<vardecl> ::= "var" <assignment>
<assignment> ::= <symbol> "=" <expr>
<varlist> ::= <symbol> [ "," <symbol> ]+

Variables must always be given a value when declared. All variables simply hold 32 bit quantities, normally interpreted as an integer. The symbol in an assignment outside of a vardecl must already have been declared.

Functions point to a string. When a procedure is called, the string is interpreted as a script (so at that time it is parsed using the language grammar). If a function is never called then it is never parsed, so it need not contain legal code if it is not called.

Strings may be enclosed either in double quotes or between { and }. The latter case is more useful for functions and similar code uses, since the brackets nest. Also note that it is legal for newlines to appear in {} strings, but not in strings enclosed by ".

<ifstmt> ::= "if" <expr> <string> [<elsepart>]
<elsepart> ::= "else" <string>
<whilestmt> ::= "while" <expr> <string> [<elsepart>]

As with functions, the strings in if and while statements are parsed and interpreted on an as-needed basis. Any non-zero expression is treated as true, and zero is treated as false. As a quirk of the implementation, it is permitted to add an "else" clause to a while statement; any such clause will always be executed after the loop exits.

<returnstmt> ::= "return" <expr>

Return statements are used to terminate a function and return a value to its caller.

<printstmt> ::= "print" <printitem> [ "," <printitem>]+
<printitem> ::= <string> | <expr>

Expressions are built up from symbols, numbers (decimal or hex integeres), and operators. The operators have precedence levels 1-4. Level 0 of expressions is the most basic, consisting of numbers or variables optionally preceded by a unary operator:

<expr0> ::= <symbol> | <number> 
            | <unaryop><expr0> 
            | "(" <expr> ")"
            | <builtincall>
<funccall> ::= <symbol> "(" [<exprlist>] ")"
<exprlist> ::= <expr> ["," <expr>]*

<number> ::= <digit>+ | "0x"<digit>+

<expr1> ::= <expr0> [<binop1> <expr0>]*
<binop1> ::= "*" | "/"

<expr2> ::= <expr1> [<binop2> <expr2>]*
<binop2> ::= "+" | "-"

<expr3> ::= <expr2> [<binop3><expr3>]*
<binop3> ::= "&" | "|" | "^" | "<<" | ">>"

<expr4> ::= <expr3> [<binop4><expr4>]*
<binop4> ::= "==" | "<>" | ">" | "<" | ">=" | "<="

<unaryop> ::= <binop1> | <binop2> | <binop3> | <binop4>

Builtin functions are defined by the runtime, as are operators. The ones listed above are merely the ones defined by default. Operators may use any of the characters =<>+-*/&|^. Note that any string of these characters is processed together, so for example a*-b is parsed as a *- b, which will cause a syntax error, rather than as a*(-b). The latter may be achieved by putting a space between the * and the -.

Note that any operator may be used as a unary operator, and in this case <op>x is interpreted as 0 <op> x for any operator <op>. This is useful for + and -, less so for other operators.

Variable Scope

Variables are dynamically scoped. For example, in:

var x=2

func printx() {
  print x
}
func myfunc() {
  var x=3
  printx()
}

invoking myfunc will cause 3 to be printed, not 2 as in statically scoped languages.

Interface to C

Environment Requirements

The interpreter is quite self-contained; the only external functions it uses are abort (called if we run out of memory), outchar (called to print a single character), and memcpy. outchar is the only one of these that is non-standard. It takes a single int as parameter and prints it as a character. This is the function the interpreter uses for output e.g. in the print statement.

Application Usage

As mentioned above, the function outchar must be defined by the application to allow for printing. The following definition will work for standard C:

#include <stdio.h>
void outchar(int c) { putchar(c); }

Embedded systems may want to provide a definition that uses the serial port or an attached display.

The application must initialize the interpreter with TinyScript_Init before making any other calls. TinyScript_Init takes two parameters: the base of a memory region the interpreter can use, and the size of that region. It returns TS_ERR_OK on success, or an error on failure. It is recommended to provide at least 2K of space to the interpreter.

If TinyScript_Init succeeds, the application may then define builtin symbols with TinyScript_Define(name, BUILTIN, (Val)func), where name is the name of the symbol in scripts and func is the C function. Technically the function should have prototype:

Val func(Val a, Val b, Val c, Val d)

However, most C compiler calling conventions are such that any C function (other than varargs ones) will work. (Certainly this is true of GCC on the Propeller). On the script side, the interpreter will supply 0 for any arguments the user does not supply, and will silently ignore arguments given beyond the fourth.

To run a script, use TinyScript_Run(script, saveStrings, topLevel). Here script is a C string, saveStrings is 1 if any variable names created in the script need to be saved in newly allocated memory -- this is necessary if the space script is stored in may later be overwritten, e.g. in a REPL loop by new commands typed by the user. topLevel is 1 if the variables created by the script should be kept after it finishes.