C# 7.x: Tuples #63
C# 7.x: Tuples #63
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BillWagner
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This PR adds the new section, "§deconstruction-expressions-new-clause Deconstruction expressions", which defines the grammar rule tuple_deconstruction_expression. However, that rule is not currently reachable, so we need to figure out where to insert it in the expressions rule hierarchy. |
Good point! There is grammar missing, and the grammar that does exist is left recursive… First let’s complete the grammar, but not make it reachable. Currently from the text a tuple_deconstruction_expression can only appear as part of a tuple deconstruction statement, so the grammar would be something like: tuple_deconstruction_statement
: tuple_deconstruction_expression '=' expression
;
tuple_deconstruction_expression
: '(' destination (',' destination)* ')'
;
destination
: type? identifier
;Left recursion is gone, and tuple_deconstruction_expression is reachable from * tuple_deconstruction_statement*, but the latter is not reachable yet. Now how to make it reachable, i.e. where does it fit into the grammar? The text suggests it is a statement_expression however all the subclauses of that are expressions which can occur elsewhere, a tuple_deconstruction_statement can only be at the statement level. (Note that while a * tuple_deconstruction_statement* may only assign to existing variables unlike assignment it cannot be a sub-expression of another expression.) The other obvious choices are: a subclause of declaration_statement – as one or more variables may be declared; a subclause of embedded_statement – on par with other statement kinds and in the hierarchy which contains assignment; or a subclause of statement itself. Subjectively I’d lean to one on the last three and away from statement_expression, YMMV and I haven't checked where Roslyn puts it as someone will know! |
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No space is allowed between the § and the name.
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| Element values are copied from the source tuple to the destination(s). Each element's position is inferred from the destination position within *destination_list*. If no variable called “_” is in scope, a *destination* with *identifier* `_` indicates a discard (§discards-new-clause), and the corresponding element is discarded rather than being copied. (Discards are discussed further below.) The destination list shall account for every element in the tuple. |
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The reference to discards-new-clause is causing the one build warning.
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See #64 for remaining questions |
| @@ -101,6 +127,20 @@ There are no predefined implicit conversions to the `char` type, so values of th | |||
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| An implicit enumeration conversion permits a *constant_expression* ([§11.20](expressions.md#1120-constant-expressions)) with any integer type and the value zero to be converted to any *enum_type* and to any *nullable_value_type* whose underlying type is an *enum_type*. In the latter case the conversion is evaluated by converting to the underlying *enum_type* and wrapping the result ([§8.3.11](types.md#8311-nullable-value-types)). | |||
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| ### §tuple-conversions-new-clause Tuple conversions | |||
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| Tuple types and expressions support a variety of conversions by "lifting" conversions of the elements into overall *tuple conversion*. For the classification purpose, all element conversions are considered recursively. For example, to have an implicit conversion, all element expressions/types shall have implicit conversions to the corresponding element types. | |||
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| *end example* | |||
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| ### §tuple-literal-expressions-new-clause Tuple literal expressions | |||
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| A tuple literal consists of two or more tuple literal elements, each of which is optionally named. | |||
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Is the intent to specify https://github.com/dotnet/csharplang/blob/main/proposals/csharp-7.1/infer-tuple-names.md separately?
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| > *end example* | ||
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| A tuple cannot be created with the `new` operator. However, the `new` operator can be used to create and initialize an array of tuple or a nullable tuple. |
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new Tuple<String, String>("a", "b") is legal. The rule is that a tuple type syntax cannot be used with the new operator.
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Closed in favor of #664 |
This (Draft) PR is not quite complete.