feat(sieve): add sieve module with Map

modules/sieve/README.md

@@ -0,0 +1,56 @@
+# sieve
+
+Generic, dependency-free collection helpers that the standard library slices and maps packages lack: mapping, filtering, reducing, keying, grouping, deduplicating, partitioning and finding, plus a small set type.
+
+`import "git.golem15.com/golem15/summercms/modules/sieve"`
+
+**Name:** A sand sieve: you shake a collection through it and keep what you need; map, filter and reduce for slices and maps.
+
+## Overview
+
+`sieve` gives plain Go slices the collection operations that WinterCMS code reaches for on Laravel collections. Every function is eager and never modifies its input, results never share a backing array with the input, callbacks run once per element in input order, and nil or empty input yields a non-nil empty result, so `encoding/json` writes `[]` rather than `null`.
+
+## Role in the framework
+
+A dependency-free leaf with no imports, so any module, plugin or application can use it without import cycles.
+
+## Features
+
+- `sieve.Map` applies a function to every element and returns the results in order; with a field accessor it is the pluck operation.
+
+## Usage
+
+```go
+package blog
+
+import "git.golem15.com/golem15/summercms/modules/sieve"
+
+type Post struct {
+	ID    int
+	Title string
+}
+
+func titles(posts []Post) []string {
+	return sieve.Map(posts, func(p Post) string { return p.Title })
+}
+```
+
+## API reference
+
+| Identifier | Description |
+|------------|-------------|
+| `sieve.Map` | Returns f applied to each element, in input order. Nil input gives a non-nil empty slice. |
+
+## Dependencies
+
+- SummerCMS modules: none.
+- Third-party: none.
+- Standard library: none (the package has no imports).
+
+## Testing
+
+```sh
+go test ./modules/sieve/...
+```
+
+The tests are in-memory and need no external services.

modules/sieve/example_test.go

@@ -0,0 +1,24 @@
+package sieve_test
+
+import (
+	"fmt"
+
+	"git.golem15.com/golem15/summercms/modules/sieve"
+)
+
+type post struct {
+	ID    int
+	Title string
+}
+
+func ExampleMap() {
+	posts := []post{
+		{ID: 1, Title: "Hello"},
+		{ID: 2, Title: "Summer"},
+		{ID: 3, Title: "Goodbye"},
+	}
+	// Pluck the titles with a field accessor.
+	titles := sieve.Map(posts, func(p post) string { return p.Title })
+	fmt.Println(titles)
+	// Output: [Hello Summer Goodbye]
+}

modules/sieve/sieve.go

@@ -0,0 +1,22 @@
+// Package sieve provides generic, dependency-free collection helpers that the
+// standard library slices and maps packages lack.
+//
+// Every function is eager and never modifies its input. Results never share a
+// backing array with the input, so changing or appending to a result leaves the
+// caller's slice unchanged. Callbacks run once per element, in input order.
+// Nil or empty input yields a non-nil empty slice or map, so encoding/json
+// writes [] or {} rather than null.
+package sieve
+
+// Map returns a new slice holding f applied to each element of s, in order.
+// f is called once per element. A nil or empty s yields a non-nil empty slice.
+//
+// Map is also the pluck operation: pass an accessor func to extract one field
+// from every element, for example the titles of a slice of posts.
+func Map[S ~[]T, T, U any](s S, f func(T) U) []U {
+	out := make([]U, 0, len(s))
+	for _, v := range s {
+		out = append(out, f(v))
+	}
+	return out
+}