ersteller/doc/2025-08-22-extract-path-params.md

ADR: Implement extractPathParams in CommonHtmxRoute

Date: 2025-08-22

Context

We use a lightweight HTTP routing based on Go 1.22+ pattern-based ServeMux. Our route paths include placeholders in braces, such as /en/events/{id}. The HtmxContext provides a GetPathParam(key string) method that uses the request’s PathValue(key) to retrieve values for path parameters.

The CommonHtmxRoute type already supports building URLs by replacing placeholders via ToUrl. However, the reverse operation — extracting path params from the template to build a []HtmxPathParam based on the current request — had an unimplemented stub: extractPathParams.

Decision

Implement extractPathParams to:

• Parse the language-specific path template for placeholders of the form {name}.
• Collect unique placeholder names in order of appearance.
• For each placeholder name, call c.GetPathParam(name) to obtain its value from the current request context.
• Return the slice of HtmxPathParam{Key: name, Value: value}.

This creates a consistent source of truth for path parameters both when generating URLs and when reading them from incoming requests.

Implementation

• File: ersteller-lib/htmx_route.go
• Function: CommonHtmxRoute.extractPathParams(c HtmxContext) []HtmxPathParam
• Approach: String scanning to find { and } pairs, trimming and deduplicating keys; for each key, append HtmxPathParam{Key: key, Value: c.GetPathParam(key)} to the results.
• No external dependencies added.

Alternatives Considered

• Using a regex like \{([^}]+)\}. Rejected to avoid introducing a regex dependency and to keep the implementation minimal and allocation-light.
• Parsing the actual request path rather than the template. Rejected because placeholder names must come from the template, and values come from GetPathParam.

Trade-offs

• The simple scanner assumes non-nested, well-formed placeholders and does not validate balanced braces beyond the first missing }. This is acceptable since templates are authored by maintainers and validated elsewhere (routing registration).

Consequences

• Callers that rely on extractPathParams now receive a correctly populated parameter list.
• No API changes; only internal behavior is completed.

Notes

• No UI controls were added/changed, so no CSS changes were required.
• Localization is unaffected since this change does not introduce user-facing strings.