Postfix Syntax

Postfix Syntax¶

Some languages are SOV — subject-object-verb — meaning the condition naturally comes before the keyword rather than after it. ForeignThon supports this with the @@ operator.

The problem¶

In English-order Python, the keyword always comes first:

if condition:
    ...

In many languages, the natural order is the opposite — the condition is stated first, then the action. Forcing English word order on these languages makes the code feel unnatural.

The solution¶

The @@ operator lets you put any keyword after its expression:

condition @@keyword:
    body

This is equivalent to:

keyword condition:
    body

Both produce identical compiled Python. @@ is purely a source-level syntax — it is processed before tokenization and never appears in the output.

Rules¶

@@ rewrites only the line it appears on — nothing else changes
Indentation follows standard Python rules, unchanged
Prefix and postfix can be mixed freely in the same file
Works for any keyword in any language pack
@@ is not valid Python syntax, so it never conflicts with existing code

Supported constructs¶

Construct	Prefix	Postfix
if	`keyword condition:`	`condition @@keyword:`
elif	`keyword condition:`	`condition @@keyword:`
while	`keyword condition:`	`condition @@keyword:`
def	`keyword name(args):`	`name(args) @@keyword:`
class	`keyword Name:`	`Name @@keyword:`
for	`keyword var in iter:`	`var @@in_kw iter @@for_kw:`

Note

for loops with postfix require two @@ operators and can be complex. Most users keep for in prefix style.

Decompile with postfix¶

When converting Python back to a foreign language, pass --postfix:

fpy decompile script.py --lang <code> --postfix

Which keywords get rewritten is controlled by the postfix_keywords list in the language pack JSON. A language that uses SVO order sets this to [] — postfix output is never forced on languages that don't need it.

"postfix_keywords": ["if", "elif", "while", "def", "class"]

Input vs output¶

Direction	Mechanism	Controlled by
Input (writing `.xx.py`)	`@@` in source	Always available for any keyword
Output (`fpy decompile --postfix`)	Pack's `postfix_keywords`	Language pack author