Skylark in Go: Implementation

This document (a work in progress) describes some of the design choices of the Go implementation of Skylark.


The scanner is derived from Russ Cox's buildifier tool, which pretty-prints Bazel BUILD files.

Most of the work happens in (*scanner).nextToken.


The parser is hand-written recursive-descent parser. It uses the technique of precedence climbing to reduce the number of productions.

In some places the parser accepts a larger set of programs than are strictly valid, leaving the task of rejecting them to the subsequent resolver pass. For example, in the function call f(a, b=c) the parser accepts any expression for a and b, even though b may legally be only an identifier. For the parser to distinguish these cases would require additional lookahead.


The resolver reports structural errors in the program, such as the use of break and continue outside of a loop.

Skylark has stricter syntactic limitations than Python. For example, it does not permit for loops or if statements at top level, nor does it permit global variables to be bound more than once. These limitations come from the Bazel project's desire to make it easy to identify the sole statement that defines each global, permitting accurate cross-reference documentation.

In addition, the resolver validates all variable names, classifying them as references to universal, global, local, or free variables. Local and free variables are mapped to a small integer, allowing the evaluator to use an efficient (flat) representation for the environment.

Not all features of the Go implementation are “standard” (that is, supported by Bazel's Java implementation), at least for now, so non-standard features such as lambda, float, and set are flag-controlled. The resolver reports any uses of dialect features that have not been enabled.


Data types

Integers: Integers are representing using big.Int, an arbitrary precision integer. This representation was chosen because, for many applications, Skylark must be able to handle without loss protocol buffer values containing signed and unsigned 64-bit integers, which requires 65 bits of precision.

Small integers (<256) are preallocated, but all other values require memory allocation. Integer performance is relatively poor, but it matters little for Bazel-like workloads which depend much more on lists of strings than on integers. (Recall that a typical loop over a list in Skylark does not materialize the loop index as an int.)

An optimization worth trying would be to represent integers using either an int32 or big.Int, with the big.Int used only when int32 does not suffice. Using int32, not int64, for “small” numbers would make it easier to detect overflow from operations like int32 * int32, which would trigger the use of big.Int.

Floating point: Floating point numbers are represented using Go's float64. Again, float support is required to support protocol buffers. The existence of floating-point NaN and its infamous comparison behavior (NaN != NaN) had many ramifications for the API, since we cannot assume the result of an ordered comparison is either less than, greater than, or equal: it may also fail.


TODO: discuss UTF-8 and string.bytes method.

Dictionaries and sets: Skylark dictionaries have predictable iteration order. Furthermore, many Skylark values are hashable in Skylark even though the Go values that represent them are not hashable in Go: big integers, for example. Consequently, we cannot use Go maps to implement Skylark's dictionary.

We use a simple hash table whose buckets are linked lists, each element of which holds up to 8 key/value pairs. In a well-distributed table the list should rarely exceed length 1. In addition, each key/value item is part of doubly-linked list that maintains the insertion order of the elements for iteration.

Struct: The skylarkstruct Go package provides a non-standard Skylark extension data type, struct, that maps field identifiers to arbitrary values. Fields are accessed using dot notation: y = s.f. This data type is extensively used in Bazel, but its specification is currently evolving.

Skylark has no class mechanism, nor equivalent of Python's namedtuple, though it is likely that future versions will support some way to define a record data type of several fields, with a representation more efficient than a hash table.


All mutable values created during module initialization are frozen upon its completion. It is this property that permits a Skylark module to be referenced by two Skylark threads running concurrently (such as the initialization threads of two other modules) without the possibility of a data race.

The Go implementation supports freezing by storing an additional “frozen” Boolean variable in each mutable object. Once this flag is set, all subsequent attempts at mutation fail. Every value defines a Freeze method that sets its own frozen flag if not already set, and calls Freeze for each value that it contains. For example, when a list is frozen, it freezes each of its elements; when a dictionary is frozen, it freezes each of its keys and values; and when a function value is frozen, it freezes each of the free variables and parameter default values implicitly referenced by its closure. Application-defined types must also follow this discipline.

The freeze mechanism in the Go implementation is finer grained than in the Java implementation: in effect, the latter has one “frozen” flag per module, and every value holds a reference to the frozen flag of its module. This makes setting the frozen flag more efficient---a simple bit flip, no need to traverse the object graph---but coarser grained. Also, it complicates the API slightly because to construct a list, say, requires a reference to the frozen flag it should use.

The Go implementation would also permit the freeze operation to be exposed to the program, for example as a built-in function. This has proven valuable in writing tests of the freeze mechanism itself, but is otherwise mostly a curiosity.

Fail-fast iterators

In some languages (such as Go), a program may mutate a data structure while iterating over it; for example, a range loop over a map may delete map elements. In other languages (such as Java), iterators do extra bookkeeping so that modification of the underlying collection invalidates the iterator, and the next attempt to use it fails. This often helps to detect subtle mistakes.

Skylark takes this a step further. Instead of mutation of the collection invalidating the iterator, the act of iterating makes the collection temporarily immutable, so that an attempt to, say, delete a dict element while looping over the dict, will fail. The error is reported against the delete operation, not the iteration.

This is implemented by having each mutable iterable value record a counter of active iterators. Starting a loop increments this counter, and completing a loop decrements it. A collection with a nonzero counter behaves as if frozen. If the collection is actually frozen, the counter bookkeeping is unnecessary. (Consequently, iterator bookkeeping is needed only while objects are still mutable, before they can have been published to another thread, and thus no synchronization is necessary.)

A consequence of this design is that in the Go API, it is imperative to call Done on each iterator once it is no longer needed.

skylark.Value interface and subinterfaces
argument passing to builtins: UnpackArgs, UnpackPositionalArgs.

Evaluation strategy: The evaluator uses a simple recursive tree walk, returning a value or an error for each expression. We have experimented with just-in-time compilation of syntax trees to bytecode, but two limitations in the current Go compiler prevent this strategy from outperforming the tree-walking evaluator.

First, the Go compiler does not generate a “computed goto” for a switch statement (Go issue 5496). A bytecode interpreter's main loop is a for-loop around a switch statement with dozens or hundreds of cases, and the speed with which each case can be dispatched strongly affects overall performance. Currently, a switch statement generates a binary tree of ordered comparisons, requiring several branches instead of one.

Second, the Go compiler‘s escape analysis assumes that the underlying array from a make([]Value, n) allocation always escapes (Go issue 20533). Because the bytecode interpreter’s operand stack has a non-constant length, it must be allocated with make. The resulting allocation adds to the cost of each Skylark function call; this can be tolerated by amortizing one very large stack allocation across many calls. More problematic appears to be the cost of the additional GC write barriers incurred by every VM operation: every intermediate result is saved to the VM's operand stack, which is on the heap. By contrast, intermediate results in the tree-walking evaluator are never stored to the heap.

frames, backtraces, errors.


skylarktest package
`assert` module
integration with Go testing.T


Discuss practical separation of code and data.