Walkabout

$ walkabout --help
walkabout is a code-generation tool to enhance struct types.
https://github.com/cockroachdb/walkabout

Usage:
  walkabout [flags]

Examples:

walkabout InterfaceName 
  Generates support code to make all struct types that implement
  the given interface walkable.

walkabout --union UnionInterface ( InterfaceName | StructName ) ...
  Generates an interface called "UnionInterface" which will be
  implemented by the named struct types, or those structs that implement
  the named interface(s).

walkabout --union UnionInterface --reachable ( InterfaceName | StructName ) ...
  As above, but also includes all types in the same package that are
  transitively reachable from the named types.  This is useful for
  refitting an entire package where the existing types may not all
  share a common interface.


Flags:
  -d, --dir string     the directory to operate in (default ".")
  -h, --help           help for walkabout
  -o, --out string     overrides the output file name
  -r, --reachable      make all transitively reachable types in the same package also
                       implement the --union interface. Only valid when using --union.
  -u, --union string   generate a new interface with the given name to be used as the
                       visitable interface.

Api

Walkabout generates two complementary APIs from existing golang sources:

• A traversal API in which a visitor function is applied to a value and all of its children. The visitor function can mutate values in-place, or apply a copy-on-mutate behavior to edit "immutable" object graphs.
• An "abstract accessor" API, which allows a visitable type to be treated as though it were simply a tree of homogeneous nodes.

Features

• Allocation-free: running a no-op visitor over a structure causes no heap allocations.
• Cycle-free: cycles are detected and broken. Note that this does not implement exactly-once behavior, but it will prevent infinite loops.
• Dependency-free: the generated code and support library depend only on built-in packages.
• Recursion-free: the core traversal code simply operates in a loop.
• Reflection-free: all type analysis is performed at generation time and reflect.Value is not used.

Use

Walkabout is driven by your existing source code. There is no special DSL, no field tags, just plain-old, idiomatic struct types. If the types that you want to make visitable already implement a common interface, you're all set. If not, don't worry, there's a flag for that.

Whenever Walkabout generates code, there is always a singular "visitable" interface in mind. This can either be a single, existing interface specified on the command line, or one can be synthesized by using the --union flag. When using the --union flag, the types specified on the command line may be interface or struct types. In either mode, we'll refer to the types specified on the command-line as "seed" types.

Walkabout will generate methods for the following "visitable" types:

• An exported struct which implements a seed interface or is a seed type.
• A slice of a visitable type.
• A pointer to a visitable type.
• An alias of a visitable type.
• Any combination of the above.
• If --reachable is used, any potentially-visitable type in the current package that is reachable from another visitable type.

Installing

go get github.com/cockroachdb/walkabout

Status

Walkabout is currently experimental and is under active development as of December 2018 with the goal of being able to refit many of CockroachDB's type hierarchies with traversal code.

Future work

• Implement support for map-valued fields.
• Implement a Parallel() decision type to allow the fields of a struct or elements of a slice to be visited concurrently.
• Override field-traversal order / filtering of fields.
• Feature flags to turn off e.g. cycle-checking, abstract accessors, etc.
• Visiting arbitrary named types that implement a seed interface (e.g. type ScalarValue int).