feat(xmlgen): built-in helper library and caller resolvers (2/4)

[sthanikan2000]

Problem

The engine in #199 renders and escapes, but a template still has to turn form data into the shapes a receiving system expects: a reference the document spreads across four elements, a date in another layout, an enum encoded as a number. With no vocabulary for that, every caller has to supply its own before rendering anything.

Changes

Nine built-in helpers — pure, deterministic, no configuration — plus an extension point for what the data cannot supply at all.

Helper	Example	Result
part	{{ part .ref "/" 0 }} on "OFF1/A/42/2026"	OFF1 — past the end gives "", not an error
split	{{ range split .codes "," }}	iterate the fields
join	{{ join .tags "-" }}	a-b-c
date	{{ date .day "2006-01-02" "1/2/06" }}	3/4/26 — a non-matching layout is an error, never a guess
decimal	{{ decimal .fob 2 }} on 1400	1400.00
lookup	{{ lookup .flag "yes" "1" "no" "0" }}	1 — unmapped values pass through
zero	{{ if zero .gain }}	true for nil, "", false, numeric zero
coalesce	{{ coalesce .a .b "n/a" }}	first value present
trim	{{ trim .name }}	surrounding whitespace removed

type ResolveFunc func(ctx context.Context, args ...any) (any, error)
type Resolvers map[string]ResolveFunc

func WithResolvers(r Resolvers) Option
func Validate(tmpl []byte, resolverNames ...string) error

• Shipping the helpers in the box is what keeps Generate(ctx, tmpl, data) sufficient for most templates.
• Resolvers are for values needing I/O — a code-list description from a database. Each registers under its own name, so a template calling one that was not supplied fails when the template is parsed, not at execution if a branch happens to reach it.
• Names are validated first because text/template's Funcs panics on a name that is not a Go identifier: a resolver called "code-list" would otherwise take down the process rather than fail the request.
• zero exists because UseNumber carries numbers as json.Number, a string underneath — so {{ if .quantity }} is true even when the quantity is 0, and a template rendering an empty-value marker for a zero would quietly render the zero instead.
• Validate checks a template without data, so a broken one fails when it is stored rather than when someone submits.

Testing

cd xmlgen
go test -race ./...
golangci-lint run -c ../.golangci.yml --timeout=5m ./...

138 subtests cumulative (38 added), race-clean, 0 lint issues. Covers each helper's edge case, resolvers receiving the caller's real Go types, ctx round-tripping, a resolver error matching both ErrResolver and the caller's own sentinel, a panicking resolver being recovered, invalid resolver names erroring rather than panicking, and call-counting across untaken branches and variable binding.

Related

Implements #189. Stacked chain — 2 of 4:

#199 engine → #201 (this) → #202 golden fixtures → #200 failure paths

[coderabbitai]

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