Binding stages and placeholder semantics

A placeholder is an unresolved obligation#

A placeholder represents a value not yet materialized at that location. It can stand for a name, a fragment, a language constant, a dependency reference, or an editorial insertion. The delimiter syntax is incidental; the binding time and authority are not.

Let $P_i$ be the binding environment for stage $i$. A staged renderer is

$$ A^{(0)}=T,\qquad A^{(i+1)}=\sigma_i(A^{(i)},P_i). $$

The final artifact is valid only if all obligations assigned to completed stages are resolved or explicitly allowed to remain as literal target-language content.

Ordering is semantic#

Suppose a global stage resolves {{component}}, and a later local stage inserts a body containing that token. The earlier global stage cannot have resolved a token that did not yet exist in the text. The design must pre-bind the body, schedule a declared later global pass, or forbid that dependency direction.

This is why the number and order of replacement passes matter. A generic “replace everything until it looks finished” loop hides both dependencies and possible nontermination.

Two replacement semantics#

Simultaneous, non-recursive substitution replaces tokens recognized in the original input of that pass. Replacement values are not scanned again during the same pass.

Sequential replacement applies an ordered list of replacements to the evolving string. A replacement can introduce text matched by a later replacement. Reordering the map can therefore change the result.

The reference model chooses explicit non-recursive passes for clarity. The inspected JCB Placeholder::update() and update_() call PHP str_replace with key/value arrays; this is an implementation-specific sequential replacement behavior, not the reference model's semantics. The paper does not equate them. J08, R12

Presence filtering is not unresolved-token validation#

JCB's action 3 filters replacement-map entries whose keys are absent from the input string before calling replacement. It does not mean “remove unknown placeholders from the file,” and it is not a complete check that every required token was resolved. J08

A portable implementation should separately validate unresolved required tokens, distinguish intended literal delimiters, and report the source of the missing binding.

Scoped and late bindings#

Component-wide bindings, view-specific bindings, reusable-code references, and language or dependency bindings can have different lifetimes. A shared environment can be reused across many artifacts while a local environment belongs to one occurrence.

Late binding is justified when the value becomes authoritative only after other structure is known. It should not become a way to hide a dependency that could have been modeled earlier.

Escaping and capture#

A string token replaced inside PHP, JSON, HTML, SQL, or a shell script crosses a syntax boundary. The renderer must know whether it is inserting an identifier, string literal, expression, or trusted code block. Generic HTML escaping is not a universal solution.

For structured targets, an AST or typed intermediate representation can make binding safer. That is a valid VDMT implementation choice even though JCB's observed mechanism often uses strings and templates.

See file emission for the concrete JCB ordering, and security for trust boundaries.