The purpose of this text format is to support:
- View Source on a WebAssembly module, thus fitting into the Web (where every source can be viewed) in a natural way.
- Presentation in browser development tools when source maps aren't present (which is necessarily the case with the Minimum Viable Product (MVP)).
- Writing WebAssembly code directly for reasons including pedagogical, experimental, debugging, optimization, and testing of the spec itself.
The text format is equivalent and isomorphic to the binary format.
The text format will be standardized, but only for tooling purposes:
- Compilers will support this format for
.Sand inline assembly. - Debuggers and profilers will present binary code using this textual format.
- Browsers will not parse the textual format on regular web content in order to implement WebAssembly semantics.
Given that the code representation is actually an Abstract Syntax Tree, the syntax would contain nested statements and expressions (instead of the linear list of instructions most assembly languages have).
There is no requirement to use JavaScript syntax; this format is not intended to be evaluated or translated directly into JavaScript.
The text format isn't uniquely representable. Multiple textual files can assemble to the same binary file, for example whitespace isn't relevant and memory initialization can be broken out into smaller pieces in the text format.
The text format is precise in that values that cannot be accurately represented in the binary format are considered invalid text. Floating-point numbers are therefore represented as hexadecimal floating-point as specified by the C99 standard, which IEEE-754-2008 section 5.12.3 also specifies. The textual format may be improved to also support more human-readable representations, but never at the cost of accurate representation.
WebAssembly currently doesn't have a final, official, text format. As detailed above the main purpose of the text format will be for human consumption, feedback from humans on readability will therefore factor into standardizing a text format.
There are, however, prototype syntaxes which are used to bring up WebAssembly: it's easier to develop using a text format than it is with a binary format, even if the ultimate WebAssembly format will be binary. Most of these prototypes use s-expressions because they can easily represent expression trees and ASTs (as opposed to CFGs) and don't have much of a syntax to speak of (avoiding syntax bikeshed discussions).
Here are some of these prototypes. Keep in mind that these aren't official, and the final official format may look entirely different:
- Prototype specification consumes an s-expression syntax.
- WAVM backend consumes compatible s-expressions.
- sexpr-wasm prototype consumes compatible s-expressions, and works closely with the V8 prototype.
- LLVM backend (the
CHECK:parts of these tests) emits compatible s-expressions. - ilwasm emits compatible s-expressions.
- wassembler consumes a different syntax, and works closely with the V8 prototype.
The binary format inherently strips names from functions, locals, globals, etc, reducing each of these to dense indices. Without help, the text format must therefore synthesize new names. However, as part of the tooling story, a lightweight, optional "debug symbol" global section may be defined which associates names with each indexed entity and, when present, these names will be used in the text format projected from a binary WebAssembly module.