initial implementation

see TODO.md for problem definition

pkg/result/TODO.md

@@ -0,0 +1,87 @@
+# result: Concurrency API — open design problem
+
+## Current API
+
+```go
+Async[T](fn func() T) <-chan Expect[T]
+AsyncOf[T](fn func() Expect[T]) <-chan Expect[T]
+Bind[T, A](fn func(A) Expect[T], arg A) func() Expect[T]
+```
+
+Typical call site:
+
+```go
+hostCh := result.AsyncOf(result.Bind(parseHost, "localhost"))
+portCh := result.AsyncOf(func() result.Expect[int] { return parsePort("8080") })
+```
+
+## The problem
+
+Every async call requires either an explicit closure or a `Bind` call. For
+single-arg library functions `Bind` works cleanly; for zero-arg and multi-arg
+functions the caller must write a closure, introducing visual noise that does
+not add information.
+
+### Why `result.Async(parsePort("8080"))` does not work
+
+`parsePort("8080")` is an eager call — it evaluates immediately on the calling
+goroutine and returns an `int`. `Async` expects `func() int`. The compiler
+rejects it. There is no way in Go to pass a call expression as a deferred
+computation without wrapping it in a closure.
+
+## Options explored
+
+### 1. `AsyncBind` / `AsyncBind2` — collapse `AsyncOf` + `Bind`
+
+```go
+func AsyncBind[T, A any](fn func(A) Expect[T], a A) <-chan Expect[T]
+func AsyncBind2[T, A, B any](fn func(A, B) Expect[T], a A, b B) <-chan Expect[T]
+```
+
+Reduces call site to `result.AsyncBind(parsePort, "8080")`.  
+**Downside:** still need `AsyncBind3`, etc. for higher arities; `Bind` itself
+becomes redundant.
+
+### 2. Higher-level combinator — `Join2`, `Join3`, …
+
+Hides channels entirely. Runs N computations concurrently, collects results,
+calls a combiner only on success:
+
+```go
+url, err := result.Join2(
+    result.Bind(parseHost, "localhost"),
+    result.Bind(parsePort, "8080"),
+    func(host string, port int) string {
+        return fmt.Sprintf("http://%s:%d", host, port)
+    },
+).Unwrap()
+```
+
+**Upside:** no channels, no closures in application code; most declarative.  
+**Downside:** loses the "start goroutines, do other work, collect later"
+pattern; still needs `Join2`/`Join3`/… per arity; heterogeneous type
+combinations explode the generic signature.
+
+### 3. Reflection / `any` variadic
+
+A single `AsyncCall(fn any, args ...any)` using `reflect.Call`.  
+**Downside:** no type safety, runtime panics on arity/type mismatch, poor IDE
+support. Not worth it.
+
+## Root constraint
+
+Go generics have no variadic type parameters. There is no way to write a
+single type-safe function that accepts an arbitrary function and its arguments.
+Every approach above hits this wall and works around it by duplicating the
+function for each arity (N = 1, 2, 3, …).
+
+## Recommended direction when revisiting
+
+Decide first which usage pattern dominates:
+
+- **"Fire and forget, collect later"** — keep channels visible; `AsyncBind` /
+  `AsyncBind2` are the minimal improvement.
+- **"Run N things, combine result"** — adopt `Join2`/`Join3`; channels
+  disappear from application code entirely.
+
+A mixed API (both families) is possible but adds surface area. Keep it small.

pkg/result/doc.go

@@ -38,16 +38,16 @@
 //
 // # Layering rule
 //
-// Reusable library code (packages under pkg/) must only *return* Expect[T] —
-// it must never call .Expect(), .Must(), or .Expectf() itself. Those methods
-// exit the current goroutine and are only safe inside a goroutine controlled
-// by [Go] or [Run].
+// The rule is simple: .Expect() is safe anywhere a boundary ([Go] or [Run])
+// owns the goroutine. In practice:
 //
-// The right split:
-//
-//   - pkg/ functions: return Expect[T] — let the caller decide how to handle it.
-//   - Application code (cmd/, HTTP handlers, …): chain .Expect() calls freely,
-//     protected by a defer result.Catch(&err) or a result.Run wrapper.
+//   - pkg/ functions that just compute and return: return Expect[T], let the
+//     caller decide how to handle it.
+//   - pkg/ functions that internally spawn goroutines via [Go] or [Run]: they
+//     own those goroutines and may freely chain .Expect() inside them. From
+//     the outside they still look like normal functions returning Expect[T].
+//   - Application code (cmd/, HTTP handlers, …): chain .Expect() freely,
+//     protected by a [Run] wrapper or defer [Catch].
 //
 // Stack traces are captured at the failure site and can be retrieved from the
 // collected error via [StackTrace].
@@ -85,6 +85,33 @@
 // runtime.Goexit which recover() cannot intercept — use [Run] or [Go] instead,
 // as they work correctly in both builds.
 //
+// # Concurrent pattern
+//
+// Combining [Async] with the boundary pattern makes concurrent code almost as
+// readable as sequential code. Fire goroutines with [Async], then collect with
+// [All] or by reading channels individually — failures surface as normal errors
+// at the boundary, with no manual WaitGroups, mutex guards, or error channels:
+//
+//	func fetchAll(urls []string) ([]string, error) {
+//	    return result.Map(urls, func(url string) string {
+//	        return fetch(url).Expect("fetch")  // happy path inside the goroutine
+//	    }).Unwrap()
+//	}
+//
+// For heterogeneous concurrent work use [AsyncOf] — it accepts functions that
+// already return Expect[T] (as library functions should), so only one .Expect()
+// per goroutine is needed at the collection site:
+//
+//	func loadConfig() (Config, error) {
+//	    hostCh := result.AsyncOf(resolveHost)  // resolveHost() Expect[string]
+//	    portCh := result.AsyncOf(resolvePort)  // resolvePort() Expect[int]
+//	    return result.Go(func() Config {
+//	        host := (<-hostCh).Expect("resolve host")
+//	        port := (<-portCh).Expect("resolve port")
+//	        return Config{Host: host, Port: port}
+//	    }).Unwrap()
+//	}
+//
 // Genuine runtime panics (nil-pointer dereferences, index out of bounds, etc.)
 // are not recovered — they still crash the program, as they should.
 package result

pkg/result/example_test.go

@@ -148,3 +148,106 @@ func Example_unwrap() {
 	// Output:
 	// 443
 }
+
+// Example_all shows result.All collecting values from concurrently running
+// goroutines. All goroutines are started before any result is read.
+func Example_all() {
+	aCh := result.Async(func() int { return parsePort("80").Must() })
+	bCh := result.Async(func() int { return parsePort("443").Must() })
+	cCh := result.Async(func() int { return parsePort("8080").Must() })
+
+	ports := result.All(aCh, bCh, cCh).Must()
+	fmt.Println(ports)
+	// Output:
+	// [80 443 8080]
+}
+
+// Example_allError shows result.All returning on the first error (in channel
+// order). Here bCh fails, so its error is returned and cCh is never read.
+func Example_allError() {
+	aCh := result.Async(func() int { return parsePort("80").Must() })
+	bCh := result.Async(func() int { return parsePort("99999").Must() })
+	cCh := result.Async(func() int { return parsePort("8080").Must() })
+
+	if err := result.All(aCh, bCh, cCh).Err(); err != nil {
+		fmt.Println("failed:", err)
+	}
+	// Output:
+	// failed: example_test.go:25: 99999 out of range
+}
+
+// Example_map shows result.Map running a function concurrently over a slice.
+// All goroutines complete and all errors are collected.
+func Example_map() {
+	ports, err := result.Map(
+		[]string{"80", "443", "8080"},
+		func(s string) int { return parsePort(s).Must() },
+	).Unwrap()
+
+	if err != nil {
+		fmt.Println("failed:", err)
+		return
+	}
+	fmt.Println(ports)
+	// Output:
+	// [80 443 8080]
+}
+
+// Example_mapError shows result.Map collecting all errors when multiple inputs
+// fail, rather than stopping at the first.
+func Example_mapError() {
+	_, err := result.Map(
+		[]string{"80", "bad", "99999"},
+		func(s string) int { return parsePort(s).Must() },
+	).Unwrap()
+
+	if err != nil {
+		fmt.Println("failed:", err)
+	}
+	// Output:
+	// failed: example_test.go:22: result.Of
+	// strconv.Atoi: parsing "bad": invalid syntax
+	// example_test.go:25: 99999 out of range
+}
+
+// Example_asyncOf shows result.AsyncOf running library functions (which return
+// Expect[T]) concurrently. Only one .Expect() per goroutine is needed — at the
+// collection site inside the boundary.
+func Example_asyncOf() {
+	hostCh := result.AsyncOf(result.Bind(parseHost, "localhost"))
+	portCh := result.AsyncOf(func() result.Expect[int] { return parsePort("8080") })
+
+	url, err := result.Go(func() string {
+		host := (<-hostCh).Expect("parse host")
+		port := (<-portCh).Expect("parse port")
+		return fmt.Sprintf("http://%s:%d", host, port)
+	}).Unwrap()
+
+	if err != nil {
+		fmt.Println("failed:", err)
+		return
+	}
+	fmt.Println(url)
+	// Output:
+	// http://localhost:8080
+}
+
+// Example_asyncOfError shows result.AsyncOf propagating a failure through the
+// channel — the error surfaces at the .Expect() call inside the boundary.
+func Example_asyncOfError() {
+	hostCh := result.AsyncOf(func() result.Expect[string] { return parseHost("") })
+	portCh := result.AsyncOf(func() result.Expect[int] { return parsePort("8080") })
+
+	_, err := result.Go(func() string {
+		host := (<-hostCh).Expect("parse host")
+		port := (<-portCh).Expect("parse port")
+		return fmt.Sprintf("http://%s:%d", host, port)
+	}).Unwrap()
+
+	if err != nil {
+		fmt.Println("failed:", err)
+	}
+	// Output:
+	// failed: parse host
+	// example_test.go:13: host must not be empty
+}

pkg/result/result.go

@@ -189,6 +189,36 @@ func Async[T any](fn func() T) <-chan Expect[T] {
 	return ch
 }
 
+// AsyncOf is like [Async] but for functions that already return Expect[T] —
+// typically library functions that follow the layering rule. This avoids the
+// double .Expect() that would otherwise be needed: one inside the goroutine to
+// unwrap, and one on the channel read to re-propagate.
+//
+//	hostCh := result.AsyncOf(resolveHost)   // resolveHost() Expect[string]
+//	portCh := result.AsyncOf(resolvePort)   // resolvePort() Expect[int]
+//	return result.Go(func() Config {
+//	    host := (<-hostCh).Expect("resolve host")
+//	    port := (<-portCh).Expect("resolve port")
+//	    return Config{Host: host, Port: port}
+//	}).Unwrap()
+func AsyncOf[T any](fn func() Expect[T]) <-chan Expect[T] {
+	ch := make(chan Expect[T], 1)
+	go func() {
+		ch <- fn()
+	}()
+	return ch
+}
+
+// Bind partially applies a single-argument function, returning a zero-argument
+// closure suitable for [AsyncOf]. This hides the closure syntax when the
+// library function takes one argument:
+//
+//	hostCh := result.AsyncOf(result.Bind(parseHost, "localhost"))
+//	portCh := result.AsyncOf(result.Bind(parsePort, "8080"))
+func Bind[T, A any](fn func(A) Expect[T], arg A) func() Expect[T] {
+	return func() Expect[T] { return fn(arg) }
+}
+
 // Go runs fn in a new goroutine and blocks until it completes, returning the
 // result as Expect[T].  It is a convenience wrapper around [Async] for the
 // common single-goroutine case.
@@ -218,6 +248,51 @@ func Run(fn func()) error {
 	}).Err()
 }
 
+// All waits for each channel in order and returns the collected values, or the
+// first error encountered. Remaining goroutines finish normally — their
+// channels are buffered so no goroutines are leaked.
+//
+// Failure detection is channel-order, not wall-clock: if ch[2] fails before
+// ch[0] finishes, you still wait for ch[0] first. In practice this rarely
+// matters — arrange channels so the ones most likely to fail come first and
+// the two orderings are equivalent.
+func All[T any](chs ...<-chan Expect[T]) Expect[[]T] {
+	out := make([]T, len(chs))
+	for i, ch := range chs {
+		r := <-ch
+		if r.err != nil {
+			return Err[[]T](r.err)
+		}
+		out[i] = r.value
+	}
+	return Ok(out)
+}
+
+// Map runs fn on each input in its own goroutine, waits for all to finish,
+// and returns the results in input order. Unlike [All], every goroutine runs
+// to completion — all errors are collected and returned together via
+// errors.Join so callers see the full failure set.
+func Map[In, Out any](inputs []In, fn func(In) Out) Expect[[]Out] {
+	chs := make([]<-chan Expect[Out], len(inputs))
+	for i, in := range inputs {
+		chs[i] = Async(func() Out { return fn(in) })
+	}
+	out := make([]Out, len(inputs))
+	var errs []error
+	for i, ch := range chs {
+		r := <-ch
+		if r.err != nil {
+			errs = append(errs, r.err)
+		} else {
+			out[i] = r.value
+		}
+	}
+	if len(errs) > 0 {
+		return Err[[]Out](errors.Join(errs...))
+	}
+	return Ok(out)
+}
+
 // Catch recovers a panic produced by [Expect.Must] and stores it in *errp.
 // For normal error propagation use [Go] or [Run] instead — they collect
 // failures from [Expect.Expect] and [Expect.Expectf] regardless of build tag.