---
title: "Structured Text"
---

# Structured Text

The PLC module's Structured Text language service for .st files and TwinCAT object files: colouring, outline, completion, hover, navigation, rename and structure checks.

**State:** modelled. The module's own language service for IEC 61131-3 Structured Text with the TwinCAT and CODESYS extensions, checked on every Structured Text region of the real TwinCAT fixtures and fuzzed, Linux only. It never claims a program compiles: that is the vendor compiler's call (see [Build with the vendor compiler](https://neuralinverse.com/docs/modules/plc/build/)).

## In `.st` and `.iecst` files

These files open in the language **Structured Text** (id `iecst`):

- **Colouring** from the theme, multi-line comments included.
- **Outline** of POUs, types, variable sections and members.
- **Completion** of variables in scope, POUs, types, keywords, members after a dot (following `EXTENDS`, `THIS^`, `SUPER^`) and named parameters in a call. `qualified_only` enumerations and global lists are offered only qualified.
- **Hover** with the declaration line and its comment.
- **Go to Definition** and **Find All References** across the open files and the workspace's `.st` files.
- **Rename** across them. It refuses collisions, shadowing and names it cannot resolve.
- **Structure checks** in Problems as you type (below).

## In TwinCAT object files

`.TcPOU`, `.TcGVL`, `.TcDUT` and `.TcIO` files open as XML with:

- an outline of the object, its methods, properties with their Get and Set bodies, and variables, at their positions in the file;
- the same structure checks on each declaration and body, reported at their exact place in the XML;
- an XML problem when the file is not well-formed.

## Structure checks

Problems lists, as you type:

- unclosed comments, strings and pragmas;
- an `END_` that matches nothing or the wrong opener;
- blocks never closed;
- malformed literals and undefined escapes;
- characters that are not Structured Text.

The checks are lexical and structural only. Types, undeclared names and semantics are the vendor compiler's call. The lexer follows IEC 61131-3 third edition plus nested comments, `{...}` pragmas and `?=`.

## Not verified

- Anything the vendor compiler checks.
- Customer projects of realistic size; Windows.