88 lines
2.4 KiB
Markdown
88 lines
2.4 KiB
Markdown
## 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:
|
|
|
|
```python
|
|
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`:
|
|
|
|
```bash
|
|
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.
|
|
|
|
```json
|
|
"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 | Postfix Syntax
|