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Positional vs. named parameters

Definition

Both forms are legal IEC 61131-3:

  • Positional: fc(a, b, c) — arguments are matched to parameters by their position in the parameter list. Concise but fragile against parameter-list refactors.
  • Named: fc(IN1 := a, IN2 := b, MODE := c) — arguments are matched by name. Verbose but stable across refactors and self-documenting.

The two forms cannot be mixed within a single call.

Syntax

Positional

result := fname ( expr1 , expr2 , ... );

Arguments must appear in the order declared on the function / FB. Trailing optional inputs may be omitted (matiec extension — the IEC standard requires all positional arguments to be present).

Named

result := fname ( param1 := expr1 , param2 := expr2 , ... );

Arguments may appear in any order. Optional inputs may be omitted. The parameter name on the left of := must match the callee’s declaration exactly (case-sensitive in matiec).

When to prefer which

SituationRecommended form
Single-argument standard function (ABS, SQRT, NOT)Positional — concise, no ambiguity
Multi-argument standard function with stable parameter list (SEL, MUX, LIMIT)Either; named for clarity in non-trivial cases
Multi-argument FB call (TON, CTU, RS)Named — the parameter list (IN/PT/Q/ET) is meaningful and the order is non-obvious to a reader
User-written FB or functionNamed — defends the call site against later parameter-list refactors
Existing call site under maintenanceMatch the surrounding style

Examples

(* Positional — concise, fine for single-arg standard FUNs *)
iAbs    := ABS(-3);
rRoot   := SQRT(2.0);
xNotted := NOT xFlag;

(* Named — multi-arg FB, documents itself *)
stepTimer(IN := xStart,
          PT := T#100ms);

(* Mixed — IEC-illegal, matiec rejects *)
(* ✗ stepTimer(xStart, PT := T#100ms);  *)

(* Both forms work; pick named for FBs by default *)
edgeRise(CLK := xButton);              (* named *)
edgeFall(xButton);                     (* positional — also legal *)

IEC reference

  • Calling functions: clause 6.6.2.1 of IEC 61131-3 third edition (2013).
  • Calling function blocks: clause 6.6.2.2.

matiec conformance

matiec accepts both forms as required. Two strict points worth flagging:

  1. No mixing within one call. matiec rejects fc(a, p := v) even though some vendor compilers accept it.
  2. Case-sensitive parameter names in the named form. stepTimer(in := x, pt := t) is rejected even though IEC identifiers are case-insensitive in general. Always match the spelling reported by library.read_block.

ForgeIEC notes

  • The MCP tool library.read_block(name) returns the canonical parameter list for any FB or function. Use it before writing a multi-arg call to look up the exact parameter spelling, especially for built-ins like TON (parameters: IN, PT, Q, ET) or for user FBs whose parameter list might have changed since the last call site was written.
  • The auto-completion in the ForgeIEC editor suggests parameter names in named form by default. If you typed a positional argument and hit Tab, the completion converts to named.