Add miter package
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136
miter/iter.go
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136
miter/iter.go
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// Package miter implements a simple iterator type, as well as helper types
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// around it.
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package miter
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import (
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"context"
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"errors"
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)
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// ErrEnd is returned from Iterator's Next method when the iterator has
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// completed.
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var ErrEnd = errors.New("end")
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// Iterator is used to visit a sequence of elements one by one.
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type Iterator[T any] interface {
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// Next will block until:
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//
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// - It returns the next item in the sequence
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// - The Context is canceled and ctx.Err() is returned
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// - ErrEnd is returned, indicating there are no more items in the sequence
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// - Some other error is returned, indicating an unexpected error occurred
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//
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// If any non-context error is returned then Next should not be called
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// again.
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Next(ctx context.Context) (T, error)
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}
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////////////////////////////////////////////////////////////////////////////////
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type iterFn[T any] func(context.Context) (T, error)
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// FromFunc wraps a function such that it implements Iterator.
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func FromFunc[T any](fn func(context.Context) (T, error)) Iterator[T] {
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return iterFn[T](fn)
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}
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func (f iterFn[T]) Next(ctx context.Context) (T, error) { return f(ctx) }
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// Error returns an Iterator which will always return the given error as the
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// error result.
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func Error[T any](err error) Iterator[T] {
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return FromFunc(func(context.Context) (T, error) {
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var zero T
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return zero, err
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})
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}
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////////////////////////////////////////////////////////////////////////////////
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// ToSlice consumes all items off the given Iterator until ErrEnd, creating and
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// returning a slice. If the Iterator ever returns any other error then that
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// error is returned, along with all consumed items so far.
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func ToSlice[T any](ctx context.Context, i Iterator[T]) ([]T, error) {
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var s []T
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for {
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v, err := i.Next(ctx)
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if errors.Is(err, ErrEnd) {
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return s, nil
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} else if err != nil {
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return s, err
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}
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s = append(s, v)
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}
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}
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type iterSlice[T any] []T
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// FromSlice wraps a slice such that it implements Iterator.
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func FromSlice[T any](s []T) Iterator[T] {
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it := iterSlice[T](s)
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return &it
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}
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func (s *iterSlice[T]) Next(context.Context) (T, error) {
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if len(*s) == 0 {
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var zero T
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return zero, ErrEnd
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}
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next := (*s)[0]
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*s = (*s)[1:]
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return next, nil
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}
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////////////////////////////////////////////////////////////////////////////////
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type iterCh[T any] <-chan T
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// FromChannel wraps a channel such that it implements Iterator.
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func FromChannel[T any](ch <-chan T) Iterator[T] {
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return iterCh[T](ch)
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}
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func (ch iterCh[T]) Next(ctx context.Context) (T, error) {
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var (
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v T
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ok bool
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)
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select {
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case v, ok = <-ch:
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if !ok {
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return v, ErrEnd
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}
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return v, nil
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case <-ctx.Done():
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return v, ctx.Err()
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}
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}
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////////////////////////////////////////////////////////////////////////////////
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// Lazily returns an Iterator which will initialize itself using the given
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// callback upon the first call to Next. If the callback returns an error then
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// this error is returned from all calls to Next.
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//
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// Lazily is useful for cases where an Iterator requires a Context to
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// initialize, but no Context is available.
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func Lazily[T any](
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fn func(ctx context.Context) (Iterator[T], error),
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) Iterator[T] {
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var i Iterator[T]
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return FromFunc(func(ctx context.Context) (T, error) {
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if i == nil {
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var err error
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if i, err = fn(ctx); err != nil {
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i = Error[T](err)
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}
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}
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return i.Next(ctx)
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})
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}
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102
miter/util.go
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102
miter/util.go
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@ -0,0 +1,102 @@
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package miter
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import (
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"context"
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"errors"
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)
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// Empty returns the empty Iterator, i.e. one which will immediately produce
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// ErrEnd.
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func Empty[T any]() Iterator[T] { return Error[T](ErrEnd) }
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// ForEach calls fn with each item read off the iterator, returning nil once
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// ErrEnd is returned from the Iterator or function. If the Iterator or function
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// return any other error then that is returned instead.
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func ForEach[T any](
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ctx context.Context, i Iterator[T], fn func(T) error,
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) error {
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for {
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v, err := i.Next(ctx)
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if errors.Is(err, ErrEnd) {
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return nil
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} else if err != nil {
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return err
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}
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if err := fn(v); errors.Is(err, ErrEnd) {
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return nil
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} else if err != nil {
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return err
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}
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}
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}
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// Map will read items from the given Iterator, pass them through the given
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// mapping function, and produce the mapped items from the returned Iterator.
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//
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// If the mapping function returns an error then that is returned from returned
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// Iterator, and no more calls should be made to it.
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func Map[T1, T2 any](i Iterator[T1], fn func(T1) (T2, error)) Iterator[T2] {
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return FromFunc(func(ctx context.Context) (T2, error) {
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v, err := i.Next(ctx)
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if err != nil {
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var zero T2
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return zero, err
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}
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return fn(v)
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})
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}
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// Concat concats all the given Iterators together into a single larger one.
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// Each Iterator will be consumed until ErrEnd in turn. Any other errors from
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// the inner Iterators will be returned as-is from the outer one.
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func Concat[T any](iters ...Iterator[T]) Iterator[T] {
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var (
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i int
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zero T
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)
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return FromFunc(func(ctx context.Context) (T, error) {
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for {
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if i >= len(iters) {
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return zero, ErrEnd
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}
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v, err := iters[i].Next(ctx)
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if errors.Is(err, ErrEnd) {
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i++
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continue
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}
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return v, err
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}
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})
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}
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// Filter returns an Iterator which will produce all items from the given
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// Iterator for which the function returns true. If the function returns any
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// error then that error is returned as-is.
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func Filter[T any](
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i Iterator[T], fn func(context.Context, T) (bool, error),
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) Iterator[T] {
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var zero T
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return FromFunc(func(ctx context.Context) (T, error) {
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for {
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v, err := i.Next(ctx)
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if err != nil {
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return zero, err
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}
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keep, err := fn(ctx, v)
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if err != nil {
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return zero, err
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}
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if keep {
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return v, nil
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}
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}
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})
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}
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