Diagnostics

ST LSP reports syntax errors in real time as you edit .st files.

How It Works

Every keystroke re-parses the document using the built-in IEC 61131-3 parser. Parse errors are converted to VS Code diagnostics and shown as:

Red squiggles in the editor
Error entries in the Problems panel (Ctrl+Shift+M)

Error Recovery

The parser uses error recovery to continue past malformed statements, so multiple errors in a file are reported at once rather than stopping at the first problem.

Example

Given an incomplete IF statement:

IF x > 0
  y := 1;
// Missing END_IF

The diagnostics panel will show:

[Error] Expected END_IF  (line 3)

Severity Levels

Severity	When
Error	Syntax violations that prevent parsing, duplicate variable declarations
Warning	Semantic issues that may indicate bugs or style problems

Warning Diagnostics

ST LSP reports warnings for several categories of issues:

Unnecessary Semicolons

Extra semicolons after control structures, in VAR blocks, or after declaration headers are flagged.

IF x > 0 THEN
  y := 1;
END_IF;  // ← Warning: Unnecessary semicolon

Diagnostic code: unnecessary-semicolon

Undefined Identifiers

References to names not found in the current scope, workspace, or standard libraries.

y := unknownVar + 1;  // ← Warning: Undefined identifier 'unknownVar'

Unknown Types

Variable declarations using a type name that cannot be resolved.

VAR
  x : NonExistentType;  // ← Warning: Unknown type: "NonExistentType"
END_VAR

Type Mismatch

Assignments or initializations where the value type is incompatible with the declared variable type (e.g., assigning a numeric expression to a BOOL, or a STRING to a numeric variable).

VAR
  flag : BOOL := 42;  // ← Warning: Type mismatch
END_VAR

Duplicate POU Names

Multiple PROGRAM, FUNCTION_BLOCK, or FUNCTION declarations with the same name in a file.

Missing Library References

A variable uses a type from a library that is not listed in the project’s library references.

Unresolved EXTENDS / IMPLEMENTS

A FUNCTION_BLOCK, INTERFACE, or STRUCT references a base type or interface that cannot be found.

Multi-Variable Declarations

Multiple variables declared on a single line (e.g., a, b, c : INT;) produce a style warning suggesting they be split into separate lines.

Diagnostic code: multi-variable-declaration