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Fift deep dive

Fift is a high-level stack-based language used for local manipulation of cells and other TVM primitives. Its primary purpose is to compile TVM assembly code into contract code as a bag-of-cells (BOC).

caution

Advanced topic notice This section covers low-level interactions with TON's implementation details. Before proceeding, ensure you have:

  • Solid experience with stack-based programming paradigms
  • Understanding of virtual machine architectures
  • Familiarity with low-level data structures

Simple arithmetic​

Use the Fift interpreter as a calculator with reverse Polish notation:

6 17 17 * * 289 + .
2023 ok

This example calculates:

  1. 17 * 17 = 289
  2. 6 * 289 = 1734
  3. 1734 + 289 = 2023

Standard output​

27 emit ."[30;1mgrey text" 27 emit ."[37m"
grey text ok
  • emit prints the Unicode character corresponding to the number on top of the stack
  • ."..." outputs a constant string

Defining functions (Fift words)​

To define a word, follow these steps:

  1. Enclose the word's effects in curly braces {}.
  2. Add a colon : after the closing brace.
  3. Specify the word's name after the colon.

First line defines a word increment that increases x by 1.

Examples:

{ x 1 + } : increment
{ minmax drop } : min
{ minmax nip } : max

Fift.fif

In TON, multiple defining words exist, not just :. They differ in behavior:

  • Active words – Operate inside curly braces {}.
  • Prefix words – Do not require a trailing space .
{ bl word 1 2 ' (create) } "::" 1 (create)  
{ bl word 0 2 ' (create) } :: :  
{ bl word 2 2 ' (create) } :: :_  
{ bl word 3 2 ' (create) } :: ::_  
{ bl word 0 (create) } : create

Fift.fif

Conditional execution​

Execute code blocks conditionally using cond:

{ { ."true " } { ."false " } cond } : ?.   4 5 = ?.  4 5 < ?.
false true  ok
{ ."hello " } execute ."world"
hello world ok

Loops​

Use loop primitives for repetitive operations:

// ( l c -- l') Removes first c elements from list l
{ ' safe-cdr swap times } : list-delete-first

GetOpt.fif

Loop word times takes two arguments - let's call them cont and n - and executes cont n times. Here list-delete-first takes continuation of safe-cdr (command deleting head from Lisp-style list), places it under c and then c times removes head from list present on stack. while/until provide conditional looping.

Comments​

Comments in Fift are defined in Fift.fif and come in two forms:

  1. Single-line comments: Start with // and continue to the end of the line
  2. Multiline comments: Start with /* and end with */
{ 0 word drop 0 'nop } :: //
{ char " word 1 { swap { abort } if drop } } ::_ abort"
{ { bl word dup "" $= abort"comment extends after end of file" "*/" $= } until 0 'nop } :: /*

Fift.fif

How Comments Work​

Fift programs are sequences of words that transform the stack or define new words. Comments must work even during word definitions, requiring them to be active words (defined with ::).

Breaking down the // definition:

  1. 0 - Pushes zero onto the stack
  2. word - Reads characters until reaching one matching the top stack value (zero is special - skips leading spaces then reads to end of line)
  3. drop - Removes the comment text from the stack
  4. 0 - Pushes zero again (number of results for :: definition)
  5. 'nop - Pushes an execution token that does nothing (equivalent to { nop })

Using Fift for defining TVM assembly codes​

x{00} @Defop NOP
{ 1 ' @addop does create } : @Defop
{ tuck sbitrefs @ensurebitrefs swap s, } : @addop
{ @havebitrefs ' @| ifnot } : @ensurebitrefs
{ 2 pick brembitrefs 1- 2x<= } : @havebitrefs
{ rot >= -rot <= and } : 2x<=
...

Asm.fif (lines order reversed)

How @Defop works​

@Defop checks available space for the opcode using @havebitrefs. If space is insufficient, it writes to another builder via @| (implicit jump).

Important: Always use x{A988} @addop instead of x{A988} s, to avoid compilation failures when space is limited.

Including cells in contracts​

You can embed large bag-of-cells into contracts:

<b 8 4 u, 8 4 u, "fift/blob.boc" file>B B>boc ref, b> <s @Defop LDBLOB

This defines an opcode that:

  1. Writes x{88} (PUSHREF) when included in the program
  2. Adds a reference to the specified bag-of-cells
  3. Pushes the cell to TVM stack when executing LDBLOB

Special features​

Ed25519 cryptography​

Fift provides built-in support for Ed25519 cryptographic operations:

  • newkeypair - Generates a private-public key pair
  • priv>pub - Derives a public key from a private key
  • ed25519_sign[_uint] - Creates a signature for given data using a private key
  • ed25519_chksign - Verifies an Ed25519 signature

TVM interaction​

  • runvmcode and similar commands - Executes TVM with a code slice taken from the stack

File operations​

  • Save BOC to file: 
    boc>B ".../contract.boc" B>file

Continue learning​