Simil Forth Language Guide

A minimal stack-based concatenative language where code is data.

Basics

Stack operations - Everything works on a stack (like an RPN calculator):

5           # Push 5 onto stack
3           # Push 3 onto stack
+           # Pop 3 and 5, push 8
.           # Pop and print: int(8)

Arithmetic:

10 5 -      # 10 - 5 = 5
6 7 *       # 6 * 7 = 42
20 4 /      # 20 / 4 = 5

Variables

Store, load, and execute:

42 :x       # Store 42 in variable x
@x .        # Load x and print: int(42)

[2 *] :double    # Store a function (list) in variable 'double'
5 !double .      # Execute double: 5 * 2 = 10

Operators:

• :name - store top of stack into variable
• @name - load variable onto stack
• !name - execute variable (must be a list)

Lists (Code as Data)

Lists are code that can be passed around:

[3 +]       # Push a list onto the stack (doesn't execute)
5 swap      # Stack: [3 +] 5
            # Now: 5 [3 +]

Comparisons & Logic

Comparisons (return 0 or 1):

5 3 >       # 1 (true)
5 3 <       # 0 (false)
5 5 ==      # 1 (true)
5 3 >=      # 1 (true)

Boolean operators:

1 1 and     # 1 (true AND true)
0 1 or      # 1 (false OR true)
1 not       # 0 (NOT true)

Control Flow

if - execute list if condition is true:

5 3 > [42 .] if     # Prints int(42) because 5 > 3
5 3 < [42 .] if     # Does nothing

ifelse - execute one of two branches:

5 3 > [100 .] [200 .] ifelse    # Prints int(100)

loop - repeat while condition is false:

0 :i
[@i 5 <]                   # Condition: i < 5
[@i . @i 1 + :i]          # Body: print i, increment
loop
# Prints: 0 1 2 3 4

Stack Manipulation

dup         # Duplicate top: 5 -> 5 5
drop        # Remove top: 5 3 -> 5
swap        # Swap top two: 5 3 -> 3 5

Complete Examples

Square function:

[dup *] :square
5 !square .         # Prints int(25)

Factorial (recursive):

[
  dup 1 ==
  [drop 1]
  [dup 1 - !fact *]
  ifelse
] :fact

5 !fact .           # Prints int(120)

Count to 10:

[1 :i]
[@i 10 >]
[@i . @i 1 + :i]
loop

Conditional greeting:

[1] :happy
@happy [100 .] [0 .] ifelse    # Prints int(100)