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State that the set of ValueTuple types provided is implementation-defined #798

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State that the set of ValueTuple types provided is implementation-defined #798

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1 change: 1 addition & 0 deletions standard/portability-issues.md
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Expand Up @@ -39,6 +39,7 @@ A conforming implementation is required to document its choice of behavior in ea
1. The behavior of the `fixed` statement if the array expression is `null` or if the array has zero elements ([§23.7](unsafe-code.md#237-the-fixed-statement)).
1. The behavior of the `fixed` statement if the string expression is `null` ([§23.7](unsafe-code.md#237-the-fixed-statement)).
1. The value returned when a stack allocation of size zero is made ([§23.9](unsafe-code.md#239-stack-allocation)).
1. The set of `System.ValueTuple<...>` types available for tuple types ([8.3.11](types.md#8311-tuple-types))

## B.4 Unspecified behavior

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2 changes: 1 addition & 1 deletion standard/types.md
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Expand Up @@ -389,7 +389,7 @@ An enumeration type is a distinct type with named constants. Every enumeration t

### 8.3.11 Tuple types

A tuple type represents an ordered, fixed-length sequence of values with optional names and individual types. The number of elements in a tuple type is referred to as its ***arity***. A tuple type is written `(T1 I1, ..., Tn In)` with n ≥ 2, where the identifiers `I1...In` are optional ***tuple element names***. This syntax is shorthand for a type constructed with the types `T1...Tn` from `System.ValueTuple<...>`, which shall be a set of generic struct types capable of expressing tuple types of any arity greater than one.
A tuple type represents an ordered, fixed-length sequence of values with optional names and individual types. The number of elements in a tuple type is referred to as its ***arity***. A tuple type is written `(T1 I1, ..., Tn In)` with n ≥ 2, where the identifiers `I1...In` are optional ***tuple element names***. This syntax is shorthand for a type constructed with the types `T1...Tn` from `System.ValueTuple<...>`, which shall be a set of generic struct types capable of expressing tuple types of any arity greater than one. The precise set of `System.ValueTuple<...>` types provided is implementation-defined.

> *Note*: There does not need to exist a `System.ValueTuple<...>` declaration that directly matches the arity of any tuple type with a corresponding number of type parameters. Instead, larger tuples can be represented with a special overload `System.ValueTuple<..., TRest>` that in addition to tuple elements has a `Rest` field containing a nested tuple value of the remaining elements. Such nesting may be observable in various ways, e.g. via the presence of a `Rest` field. *end note*
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@KalleOlaviNiemitalo KalleOlaviNiemitalo Jun 5, 2023 •
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Reading this made me worry about possible conflicts in overloaded methods, if one method takes an (int, int, int, int, int, int, int, char, char) parameter and another overload takes (int, int, int, int, int, int, int, (char, char)) -- does the compiler translate both of those to ValueTuple<int, int, int, int, int, int, int, ValueTuple<char, char>>? But no, the latter becomes ValueTuple<int, int, int, int, int, int, int, ValueTuple<ValueTuple<char, char>>> instead. Here, TRest is a ValueTuple<T1> with only one type argument, and does not correspond to any C# tuple type. The wording "a nested tuple value" is a bit misleading, then, as it could be taken to mean an instance of a tuple type.

Perhaps "shall be a set of generic struct types capable of expressing tuple types of any arity greater than one" should also be changed to require ValueTuple<T1>, at least in those implementations that translate large-arity tuple types to System.ValueTuple<..., TRest>. If the implementation does generate a dedicated generic type for tuple types of each arity, instead of using TRest, then I suppose it need not support ValueTuple<T1>, because:

  • The non-generic struct ValueTuple and its public static ValueTuple<T1> Create\<T1>(T1 item1) method is not part of Annex C Standard library.
  • struct ValueTuple<T1> is part of Annex C Standard library, but its omission can be excused because "The precise set of System.ValueTuple<...> types provided is implementation-defined."

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@KalleOlaviNiemitalo – I see your point but rather than trying to improve the wording I would suggest a different tack.

The C# Standard does not specify a “wire format” for interoperability of binary data, or how arrays should be stored, and a myriad of other things. Why is it specifying how a tuple should be stored? Why even mention ValueTuple? Are these not implementation details of the kind the Standard doesn’t usually cover?

Rather than add a sentence to the end of the paragraph, and the following note, why not delete ”This syntax is shorthand for … arity greater than one” instead?


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