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Spec: Refine the opt rule #246

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Merged
chenyan-dfinity merged 3 commits into from
Aug 30, 2021
Merged

Spec: Refine the opt rule #246

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SpeC: Refine the opt rule
nomeata Jun 16, 2021
36d3018
null <: t'
chenyan-dfinity Aug 29, 2021
e1f7a3d
Merge branch 'master' into joachim/fix-opt-rule
chenyan-dfinity Aug 29, 2021
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19 changes: 12 additions & 7 deletions spec/Candid.md
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Original file line number Diff line number Diff line change
Expand Up @@ -791,7 +791,7 @@ not (<datatype> <: opt <datatype'>)
---------------------------------
opt <datatype> <: opt <datatype'>

not (null <: <datatype>)
not (null <: <datatype'>)
not (<datatype> <: <datatype'>)
---------------------------------
opt <datatype> <: opt <datatype'>
Expand Down Expand Up @@ -915,7 +915,7 @@ C[vec <t> <: vec <t'>]( vec { <v>;* } ) = vec { C[<t> <: <t'>](<v>);* }

#### Options

The null type coerces into any option type:
The null type and the reserved type coerce into any option type:
```
C[null <: opt <t>](null) = null
```
Expand All @@ -927,15 +927,20 @@ C[opt <t> <: opt <t'>](opt <v>) = opt C[<t> <: <t'>](v) if <t> <: <t'>
C[opt <t> <: opt <t'>](opt <v>) = null if not(<t> <: <t'>)
```

NOTE: These rules above imply that a Candid decoder has to be able to decide the subtyping relation at runtime.
Coercing a non-null, non-optional and non-reserved type at an option type treats it as an optional value, if it has a suitable type:
```
C[<t> <: opt <t'>](<v>) = opt C[<t> <: <t'>](v) if not (null <: <t'>) and <t> <: <t'>
```

Coercing a non-null, non-optional and non-reserved value at an option type treats it as an optional value:
Any other type goes to `null`:
```
C[<t> <: opt <t'>](<v>) = opt C[<t> <: <t'>](v) if not (null <: <t>) and <t> <: <t'>
C[<t> <: opt <t'>](_) = null if not (null <: <t>) and not (<t> <: <t'>)
C[reserved <: opt <t'>](_) = null
C[reserved <: opt <t>](_) = null
C[<t> <: opt <t'>](_) = null if not (null <: <t'>) and not (<t> <: <t'>)
C[<t> <: opt <t'>](_) = null if null <: <t'> and not (null <: <t>)
```

NOTE: These rules above imply that a Candid decoder has to be able to decide the subtyping relation at runtime.

#### Records

In the following rule:
Expand Down