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title URLFor & ID
slug /urlfor
sidebar_position 8

URLFor & ID Generation

structpages provides type-safe helpers for generating URLs and DOM identifiers that stay in sync with the route tree:

  • URLFor(ctx, page, params) — build a URL for a page from its type.
  • Ref("Parent.Field") — string reference for cases the static type lookup can't handle.
  • ID(ctx, Page.Method) — raw HTML id attribute for a page component.
  • IDTarget(ctx, Page.Method) — #-prefixed CSS selector for HTMX hx-target.

All are validated by the structpages-lint analyzer. The rule of thumb: never write an in-app URL as a string literal — resolve it by page type so the literal can't drift when routes move.

URLFor

structpages.URLFor(ctx, page, args...) returns (string, error). Templ attribute values accept (string, error) directly:

<a href={ structpages.URLFor(ctx, MyPage{}) }>Link</a>
<a href={ structpages.URLFor(ctx, DetailPage{}, map[string]any{"itemId": item.ID}) }>Detail</a>
<form action={ structpages.URLFor(ctx, SavePage{}, map[string]any{"itemId": item.ID}) } method="POST">

The recommended shape is two arguments: URLFor(ctx, page, params) with params as a map[string]any. It's explicit at the call site, survives route changes, and fills both path and query placeholders by name. Positional and key/value-pair forms also work but are easier to misalign during refactors.

form shape use when
bare typed page URLFor(ctx, MyPage{}, params) the type is mounted exactly once
typed chain URLFor(ctx, []any{Parent{}, Leaf{}}, params) same leaf type mounted under multiple parents — parent disambiguates
chain + URL fragment URLFor(ctx, []any{Parent{}, Leaf{}, "?tab={t}"}, params) need to append a query template or path suffix
string (auto-Ref) URLFor(ctx, "Parent.Field", params) can't import the page type (cross-package cycle); top-level strings only — strings inside []any are URL fragments
Ref by qualified name URLFor(ctx, Ref("Parent.Field"), params) explicit form of the string sugar above

Always strict

A bare type that matches multiple mounted nodes errors instead of silently picking one. The error lists every match and recommends the chain form. There is no opt-out — silent first-match is always wrong, so disambiguating at the call site is mandatory:

type root struct {
    Components componentsRoot `route:"/components Components"`
    Patterns   patternsRoot   `route:"/patterns Patterns"`
}
type componentsRoot struct { Detail entryPage `route:"/{slug} Component"` }
type patternsRoot   struct { Detail entryPage `route:"/{slug} Pattern"` }

// Bare URLFor errors — entryPage matches two nodes.
url, err := structpages.URLFor(ctx, entryPage{}, map[string]any{"slug": "button"})

// Chain anchors at the parent struct; descends into the entryPage child.
url, err := structpages.URLFor(ctx,
    []any{componentsRoot{}, entryPage{}},
    map[string]any{"slug": "button"})
// → "/components/button"

Chain semantics: inside []any{...}, leading typed values form a chain through the page tree; each subsequent typed value descends into a child of that type (must be unique among siblings). Once a string appears, no more typed values are allowed; remaining strings concat literally to the pattern. A typed value after a string fragment errors at runtime.

Query strings

Pass a []any whose trailing strings form the URL template; the map[string]any fills both path and query placeholders:

url, err := structpages.URLFor(ctx,
    []any{MyList{}, "?page={page}&q={q}"},
    map[string]any{"page": pageNum, "q": query},
)

Page groups resolve to their index

A page group is never served at its bare path — ServeMux matches only its subtree, and the bare path 307-redirects to add the trailing slash. So URLFor on a page group returns its index child's URL (the /{$} route) with the canonical trailing slash:

href, err := structpages.URLFor(ctx, sectionRoot{})
// → "/section/"   (not "/section", which would 307)

Link a page group by its type and the URL serves a 200 directly. Don't hand-append a trailing slash, and don't link to the slashless form. Leaf pages return their own bare path unchanged.

Params auto-fill from the current request

Unfilled placeholders that match path params from the current request's route are filled automatically — you only pass what differs:

// Inside a handler for /orgs/{org}/products/{productId}:
href, err := structpages.URLFor(ctx, siblingPage{})
// → "/orgs/acme/products/p-123/sibling"  (params inherited)

Only the current route's params auto-fill; sibling routes with different param names do not.

Ref

When the target page can't be referenced by static type — a cross-package import would cycle, or a Go type alias collapses two routes onto one reflect.Type — use Ref (a string type):

url, err := structpages.URLFor(ctx, structpages.Ref("Admin.Settings"))

Ref resolves by field-name path in the page tree. The anchor segment matches a top-level node if one has that name, otherwise any uniquely-named node anywhere in the tree. An ambiguous anchor errors — qualify it with a parent segment. Ref strings are validated by structpages-lint (including refs stored in struct fields, e.g. a nav table), so a lint-passing Ref resolves at runtime.

ID and IDTarget

For HTMX you need the server-rendered id attribute and the client-side hx-target selector to agree. Generate both from the same method reference:

<div id={ structpages.ID(ctx, index.TodoList) }>
    @p.TodoList(props.Todos)
</div>

<form hx-post={ structpages.URLFor(ctx, addTodo{}) }
      hx-target={ structpages.IDTarget(ctx, index.TodoList) }>

ID(ctx, index.TodoList) returns the page's full field-name path joined with the method — "index-todo-list" for a top-level page, "admin-users-todo-list" when nested; IDTarget prepends #. Plain strings pass through both functions unchanged — IDTarget("body") is "body", not "#body". The full id scheme, multi-mount disambiguation, and the swap loop are covered in HTMX Integration.

Validation: no dangling URLs in production

structpages-lint is the primary guard — it statically validates URLFor/Ref calls, params, and hard-coded routes in CI. For what static analysis can't see (URLs assembled from runtime data, refs behind dynamic dispatch), add a boot-time inventory that kills the startup with the list of what's dangling:

func validateURLs(sp *structpages.StructPages) error {
    var errs []error
    check := func(label string, gen func() (string, error)) {
        if _, err := gen(); err != nil {
            errs = append(errs, fmt.Errorf("%s: %w", label, err))
        }
    }
    check("components detail", func() (string, error) {
        return sp.URLFor([]any{componentsRoot{}, entryPage{}}, map[string]any{"slug": "sample"})
    })
    check("admin settings", func() (string, error) {
        return sp.URLFor(structpages.Ref("Admin.Settings"))
    })
    return errors.Join(errs...)
}

Call it from main after Mount (fail the boot) and from a one-line test (coverage in CI). See examples/url-validation/ for the runnable pattern.

Plain strings outside templ attributes

Templ attribute expressions take (string, error) directly — no wrapper needed. The exception, still inside templ, is a context that needs a plain string, like templ.Attributes map values; use a small must helper for those (and only those):

func must[T any](v T, err error) T {
    if err != nil { panic(err) }
    return v
}
@PrimaryButton(templ.Attributes{
    "hx-get": must(structpages.URLFor(ctx, UserNewModal{})),
}) { + New User }