design: add xml stream design doc

See #19480

Change-Id: I592bac59460e552298cb5355cce3da31257a338e
Reviewed-on: https://go-review.googlesource.com/37993
Reviewed-by: Ian Lance Taylor <[email protected]>

design/19480-xml-stream.md

@@ -0,0 +1,153 @@
+# Proposal: XML Stream
+
+Author(s): Sam Whited <[email protected]>
+
+Last updated: 2017-03-09
+
+Discussion at https://golang.org/issue/19480
+
+
+## Abstract
+
+The `encoding/xml` package contains an API for tokenizing an XML stream, but no
+API exists for processing or manipulating the resulting token stream.
+This proposal describes such an API.
+
+
+## Background
+
+The [`encoding/xml`][encoding/xml] package contains APIs for tokenizing an XML
+stream and decoding that token stream into native data types.
+Once unmarshaled, the data can then be manipulated and transformed.
+However, this is not always ideal.
+If we cannot change the type we are unmarshaling into and it does not match the
+XML format we are attempting to deserialize, eg. if the type is defined in a
+separate package or cannot be modified for API compatibility reasons, we may
+have to first unmarshal into a type we control, then copy each field over to the
+original type; this is cumbersome and verbose.
+Unmarshaling into a struct is also lossy.
+As stated in the XML package:
+
+> Mapping between XML elements and data structures is inherently flawed:
+> an XML element is an order-dependent collection of anonymous values, while a
+> data structure is an order-independent collection of named values.
+
+This means that transforming the XML stream itself cannot necessarily be
+accomplished by deserializing into a struct and then reserializing the struct
+back to XML; instead it requires manipulating the XML tokens directly.
+This may require re-implementing parts of the XML package, for instance, when
+renaming an element the start and end tags would have to be matched in user code
+so that they can both be transformed to the new name.
+
+To address these issues, an API for manipulating the token stream itself, before
+marshaling or unmarshaling occurs, is necessary.
+Ideally, such an API should allow for the composition of complex XML
+transformations from simple, well understood building blocks.
+The transducer pattern, widely available in functional languages, matches these
+requirements perfectly.
+
+Transducers (also called, transformers, adapters, etc.) are iterators that
+provide a set of operations for manipulating the data being iterated over.
+Common transducer operations include Map, Reduce, Filter, etc. and these
+operations are are already widely known and understood.
+
+
+## Proposal
+
+The proposed API introduces two concepts that do not already exist in the
+`encoding/xml` package:
+
+```go
+// A Tokenizer is anything that can decode a stream of XML tokens, including an
+// xml.Decoder.
+type Tokenizer interface {
+	Token() (xml.Token, error)
+	Skip() error
+}
+
+// A Transformer is a function that takes a Tokenizer and returns a new
+// Tokenizer which outputs a transformed token stream.
+type Transformer func(src Tokenizer) Tokenizer
+```
+
+Common transducer operations will also be included:
+
+
+```go
+// Inspect performs an operation for each token in the stream without
+// transforming the stream in any way.
+// It is often injected into the middle of a transformer pipeline for debugging.
+func Inspect(f func(t xml.Token)) Transformer {}
+
+// Map transforms the tokens in the input using the given mapping function.
+func Map(mapping func(t xml.Token) xml.Token) Transformer {}
+
+// Remove returns a Transformer that removes tokens for which f matches.
+func Remove(f func(t xml.Token) bool) Transformer {}
+```
+
+Because Go does not provide a generic iterator concept, this (and all
+transducers in the Go libraries) are domain specific, meaning operations that
+only make sense when discussing XML tokens can also be included:
+
+```go
+// RemoveElement returns a Transformer that removes entire elements (and their
+// children) if f matches the elements start token.
+func RemoveElement(f func(start xml.StartElement) bool) Transformer {}
+```
+
+
+## Rationale
+
+Transducers are commonly used in functional programming and in languages that
+take inspiration from functional programming languages, including Go.
+Examples include [Clojure transducers][clojure/transducer], [Rust
+adapters][rust/adapter], and the various "Transformer" types used throughout Go,
+such as in the [`golang.org/x/text/transform`][transform] package.
+Because transducers are so widely used (and already used elsewhere in Go), they
+are well understood.
+
+
+## Compatibility
+
+This proposal introduces two new exported types and 4 exported functions that
+would be covered by the compatibility promise.
+A minimal set of Transformers is proposed, but others can be added at a later
+date without breaking backwards compatibility.
+
+
+## Implementation
+
+A version of this API is already implemented in the
+[`mellium.im/xmlstream`][xmlstream] package.
+If this proposal is accepted, the author volunteers to copy the relevant parts
+to the correct location before the 1.9 (or 1.10, depending on the length of this
+proposal process) planning cycle closes.
+
+
+## Open issues
+
+- Where does this API live?
+  It could live in the `encoding/xml` package itself, in another package (eg.
+  `encoding/xml/stream`) or, temporarily or permanently, in the subrepos:
+  `golang.org/x/xml/stream`.
+- A Transformer for removing attributes from `xml.StartElement`'s was originally
+  proposed as part of this API, but its implementation is more difficult to do
+  efficiently since each use of `RemoveAttr` in a pipeline would need to iterate
+  over the `xml.Attr` slice separately.
+- Existing APIs in the XML package such as `DecodeElement` require an
+  `xml.Decoder` to function and could not be used with the Tokenizer interface
+  used in this package.
+  A compatibility API may be needed to create a new Decoder with an underlying
+  tokenizer.
+  This would require that the new functionality reside in the `encoding/xml`
+  package.
+  Alternatively, general Decoder methods could be reimplemented in a new package
+  with the Tokenizer API.
+
+
+[encoding/xml]: https://golang.org/pkg/encoding/xml/
+[clojure/transducer]: https://clojure.org/reference/transducers
+[rust/adapter]: https://doc.rust-lang.org/std/iter/#adapters
+[transform]: https://godoc.org/golang.org/x/text/transform
+[xmlstream]: https://godoc.org/mellium.im/xmlstream