5.4.2.16 On...GoSub Statement

Note

This section describes the implementation of MS-VBAL §5.4.2.16 On...GoSub Statement.

Syntax

AST node	Instruction kind	Notes
OnGoSubStatementNode	`GoSubTable`	Lowering resolves the statement's labels into Instruction`.Targets`.

An On expression GoSub label, ... statement is represented by OnGoSubStatementNode, and lowers to the InstructionKind GoSubTable (RD-VBAL §3.5.2 Instruction).

OnGoSubStatementNode holds its targets in a Labels list. Each target is an expression naming or numbering a label or line, with no static link to the label node: the same convention as GoSub (RD-VBAL §3.4.2 Simple Statements).

OnGoToStatementNode and OnGoSubStatementNode are separate node types, rather than one shared node with a Kind.

Static Semantics

Lowering resolves each label in the list into Instruction.Targets (RD-VBAL §3.5.3 Lowering Block Statements).

Condition	Result
The procedure defines the label.	Its `Targets` entry is the label's offset.
The procedure does not define the label, e.g. `Missing` in `On n GoSub A, Missing`.	Lowering reports VBC09309 Label not defined for it. Each undefined label in the list is reported on its own.

Runtime Semantics

GoSubTable performs the same selector algorithm as JumpTable (RD-VBAL §5.4.2.13 On...GoTo Statement), and in addition pushes a resumption point on a successful branch:

The selector expression is evaluated once.
The evaluated value is Let-coerced to Integer. Let n be the coerced value.
If n is zero, or greater than the number of labels, execution falls through to the next instruction without branching.
Otherwise, if n is negative or greater than 255, run-time error 5 is raised.
Otherwise, the offset right after the On…GoSub is pushed onto the activation's GoSub Resumption List, and execution branches to the n'th label (1-based) in Instruction.Targets.

The push in step 5 is the same push a bare GoSub performs (RD-VBAL §5.4.2.14 GoSub Statement). A later Return pops it (RD-VBAL §5.4.2.15 Return Statement).

Condition	Run-time error
n is negative, or greater than 255 (step 4).	5 — Invalid procedure call or argument

A label with no statement after it, including the end-label of the lexically enclosing procedure declaration, resolves to the offset right past the last instruction. A branch to it completes the activation as if execution had reached the end of the procedure body (RD-VBAL §3.5.1 InstructionList).

Implementation

RDCore.Runtime.Execution.ProcedureExecutor dispatches GoSubTable instructions, in the code path it shares with JumpTable.
RDCore.Runtime.Execution.JumpTableEvaluator evaluates the selector and Let-coerces it to Integer (steps 1 and 2), by calling VBNumericLetCoercionTypeRuntimeSemantics directly, bypassing the let-coercion provider.
The push in step 5 goes through CallStackFrame.PushGoSubReturn.