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Fix value definition text format ambiguities #729

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Fix value definition text format ambiguities #729

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24 changes: 17 additions & 7 deletions design/mvp/Explainer.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2431,9 +2431,9 @@ Components may define values in the value index space using following syntax:
value ::= (value <id>? <valtype> <val>)
val ::= false | true
| <core:i64>
| <f32canon>
| <f64canon>
| nan
| '<core:stringchar>'
| "<core:stringchar>"
| <core:name>
| (record <val>+)
| (variant <labellit> <val>?)
Expand All @@ -2446,12 +2446,22 @@ val ::= false | true
| (map <entryval>*) 🗺️
| (binary <core:datastring>)
entryval ::= (entry <val> <val>) 🗺️
f64canon ::= <core:f64> without the `nan:0x` case.
fNcanon ::= <core:fN> excluding the `-nan` and `nan:0x` cases
```
where [`core:i64`], [`core:f64`], [`core:stringchar`] and [`core:datastring`]
are as defined by the Core WebAssembly text format.
where [`core:fN`], [`core:stringchar`] and [`core:datastring`] are as defined by
the Core WebAssembly text format.

The validation rules for `value` require the `val` to match the `valtype`.
The validation rules for `value` first reject any `valtype` that transitively
contains a type constructor that doesn't have a corresponding `val` constructor.
Currently this rejects `own`, `borrow`, `future`, `stream` and `error-context`.
In particular, `(option (stream u8))` is rejected, even if the `none` case has a
`val` constructor. In the future, `val` may be extended to support more cases.

Next, validation requires the given `val` to match the `valtype`. In particular:
* `fN` only matches `fNcanon`
* `sN` and `uN` only match `core:i64` integers that fall within their natural
signed range without wrapping (e.g., `(value s8 128)` and `(value u8 -1)`
would be rejected)

The `(binary ...)` expression form provides an alternative syntax allowing the binary contents
of the value definition to be written directly in the text format, analogous to data segments,
Expand Down Expand Up @@ -3345,7 +3355,7 @@ For some use-case-focused, worked examples, see:
[`core:id`]: https://webassembly.github.io/spec/core/text/values.html#text-id
[`core:externidx`]: https://webassembly.github.io/spec/core/text/modules.html#text-externidx
[`core:i64`]: https://webassembly.github.io/spec/core/text/values.html#integers
[`core:f64`]: https://webassembly.github.io/spec/core/text/values.html#floating-point
[`core:fN`]: https://webassembly.github.io/spec/core/text/values.html#floating-point
[`core:stringchar`]: https://webassembly.github.io/spec/core/text/values.html#text-string
[`core:name`]: https://webassembly.github.io/spec/core/text/values.html#text-name
[`core:module`]: https://webassembly.github.io/spec/core/text/modules.html#text-module
Expand Down
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