BITSY.md

Bitsy

Bitsy aims to be the best language to implement when writing your first compiler or interpreter. It is a resource for programmers learning about language design.

A secondary goal of Bitsy is to look familiar to most imperative programmers. In contrast to other languages often used to teach language implementation, such as BF or Lisp, Bitsy is comparable, at first glance, to "typical" programming languages.

A non-goal of Bitsy is to be useful itself as a language. It is only just possible to write a few useful programs in Bitsy, such as calculating factorials or prime numbers. Bitsy qua Bitsy might be useful as teaching tool if used as an early introduction to programming for new students, such as those in elementary Math.

Example

Below is a sample Bitsy program which prints the fibonacci sequence to a given number of terms.

{ Print the fibonacci sequence to a given number of terms }
BEGIN
  READ fib_count
  this_fib = 1

  LOOP
    IFN fib_count - 1
      BREAK
    END

    PRINT last_fib

    next_fib = this_fib + last_fib
    last_fib = this_fib
    this_fib = next_fib

    fib_count = fib_count - 1
  END
END

The BitsySwift repository has some additional samples.

Bitsy Language Definition

While the hope of this repo is to codify the definition of Bitsy in a series of runnable tests, it is also useful to define the language informally using words and examples. For a formal definition of the Bitsy grammar, see GRAMMAR.txt

Program Definition

Bitsy programs are always contained in one file with a .bitsy extension. All programs start with the BEGIN keyword and are terminated with the END keyword. The smallest valid program is:

BEGIN
END

A Bitsy program may have an arbitrary amount of whitespace or comments before the BEGIN keyword or after the final END, but the executable body of the program exists completely between these keywords.

Comments

Bitsy supports block style code comments. Comments begin with the { character and end with }. Any combination of characters may be present between the brackets. Comment blocks may span multiple lines. Comment blocks can not be nested. Comments can appear before the BEGIN keyword or after the program terminating END keyword.

{ This is the Bitsy null program }
BEGIN
  {
    It does:
     * Nothing
  }
END
{But it is valid!}

Data types

Bitsy has only one data type: signed integers. The range of Bitsy's integer type is currently not defined. It is suggested to use the largest signed range possible for the architecture your implementation targets.

Arithmetic

Bitsy supports the +, -, *, /, and % infix operators for addition, subtraction, multiplication, division (integer), and modulus. Standard precedence rules are used. Expressions may also be prepended with + or - operator for expressing signed-ness. Arithmetic operators may not be used in succession. For example, 2 - -2 is an error; it does not evaluate to 4. Similarly, 2 * -2 is wrong. Parentheses can be used to alter the order of operations as expected. Parentheses can be nested, but must be balanced.

Variables

Variables in Bitsy consist of letters and underscores. Variables are case sensitive. There is no limit to the number of characters in a Bitsy variable, nor are there any rules about how they are named.

No initialization or declaration of variables is required. If a variable is accessed before first being assigned its value is 0.

Variable assignment is performed using the = infix operator. The variable name to the left of the operator is assigned to the evaluated expression on the right.

I/O

Bitsy has two built in keywords for user input and output.

The PRINT keyword is used for output. It is followed by whitespace and any valid expression and the result is output to stdout followed by a single newline (\n).

THE READ keyword is used to take input from the user. It is followed by whitespace and a variable name. When a Bitsy program evaluates a READ command, it pauses to take input from the user and reads that input as an integer value. The value received is loaded into the variable specified. Any value previously stored in that variable is overwritten. Input that contains any non-digit character ('0'-'9') is interpreted as 0.

{ Print 10x the user's input }
BEGIN
  READ input
  PRINT 10*input
END

Conditionals

Despite having no boolean data type, Bitsy has three conditional statements: IFP, IFZ, and IFN. Each statement is followed by an expression, an optional ELSE, and a closing END. The statement proceeds based the evaluation of the expression.

An IFP statement executes the conditional block if the expression evaluates to a positive value. An IFZ statement executes the conditional block if the expression evaluates to exactly zero. An IFN statement executes the conditional block if the expression evaluates to a negative value.

Loops

Bitsy has only one loop construct: LOOP...END. Everything within a loop-end block is repeated indefinitely. A loop is exited with the BREAK keyword. When BREAK is evaluated, control moves from the break statement to the terminating END keyword. LOOP blocks may be nested. A BREAK exits only the inner most loop. The behavior of a BREAK keyword outside a loop block is currently undefined.

{ Count by 2's  to 100 }
BEGIN
  LOOP
    IFZ 102 - count
      BREAK
    END

    PRINT count
    count = count + 2
  END
END