ucl/cmdlang/streams.go

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package cmdlang
import (
"errors"
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"fmt"
"io"
)
// stream is an object which returns a collection of objects from a source.
// These are used to create pipelines
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//
// The stream implementation can expect close to be called if at least one next() call is made. Otherwise
// closableStream cannot assume that close will be called (the pipe may be left unconsumed, for example).
//
// It is the job of the final iterator to call close. Any steam that consumes from another stream must
// implement this, and call close on the parent stream.
type stream interface {
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object
// next pulls the next object from the stream. If an object is available, the result is the
// object and a nil error. If no more objects are available, error returns io.EOF.
// Otherwise, an error is returned.
next() (object, error)
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close() error
}
// forEach will iterate over all the items of a stream. The iterating function can return an error, which will
// be returned as is. A stream that has consumed every item will return nil. The stream will automatically be closed.
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func forEach(s stream, f func(object, int) error) (err error) {
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defer s.close()
var sv object
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i := 0
for sv, err = s.next(); err == nil; sv, err = s.next() {
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if err := f(sv, i); err != nil {
return err
}
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i += 1
}
if !errors.Is(err, io.EOF) {
return err
}
return nil
}
// asStream converts an object to a stream. If t is already a stream, it's returned as is.
// Otherwise, a singleton stream is returned.
func asStream(v object) stream {
if s, ok := v.(stream); ok {
return s
}
return &singletonStream{t: v}
}
type emptyStream struct{}
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func (s *emptyStream) String() string {
return "(nil)"
}
func (s emptyStream) next() (object, error) {
return nil, io.EOF
}
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func (s emptyStream) close() error { return nil }
type singletonStream struct {
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t object
consumed bool
}
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func (s *singletonStream) String() string {
return s.t.String()
}
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func (s *singletonStream) Truthy() bool {
return !s.consumed
}
func (s *singletonStream) next() (object, error) {
if s.consumed {
return nil, io.EOF
}
s.consumed = true
return s.t, nil
}
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func (s *singletonStream) close() error { return nil }
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type listIterStream struct {
list []object
cusr int
}
func (s *listIterStream) String() string {
return fmt.Sprintf("listIterStream{list: %v}", s.list)
}
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func (s *listIterStream) Truthy() bool {
return len(s.list) > s.cusr
}
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func (s *listIterStream) next() (o object, err error) {
if s.cusr >= len(s.list) {
return nil, io.EOF
}
o = s.list[s.cusr]
s.cusr += 1
return o, nil
}
func (s *listIterStream) close() error { return nil }
type mapFilterStream struct {
in stream
mapFn func(x object) (object, bool)
}
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func (ms mapFilterStream) String() string {
return fmt.Sprintf("mapFilterStream{in: %v}", ms.in)
}
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func (ms mapFilterStream) Truthy() bool {
return true // ???
}
func (ms mapFilterStream) next() (object, error) {
for {
u, err := ms.in.next()
if err != nil {
return nil, err
}
t, ok := ms.mapFn(u)
if ok {
return t, nil
}
}
}
func (ms mapFilterStream) close() error {
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return ms.in.close()
}