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| − | == DIP23 Counter Proposal: Fixing properties and optional parens. ==
| + | #REDIRECT [[DIP24]] |
| − | {| class="wikitable"
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| − | !Title:
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| − | !'''Fixing properties and optional parens. Only those. In a general and straightforward way.'''
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| − | |-
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| − | |DIP:
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| − | |23 (counter proposal)
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| − | |-
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| − | |Version:
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| − | |1
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| − | |-
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| − | |Status:
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| − | |Draft
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| − | |-
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| − | |Created:
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| − | |2013-02-06
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| − | |-
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| − | |Last Modified:
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| − | |2013-02-06
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| − | |-
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| − | |Author:
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| − | |Timon Gehr
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| − | |-
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| − | |Links:
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| − | |
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| − | |}
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| − | | |
| − | == Motivation ==
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| − | | |
| − | The DIP23 draft does not address all requests and only deals with a subset of cases. Furthermore, it is specified by example and has too much emphasis on special cases instead of general and implementable rules.
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| − | | |
| − | == Optional parens ==
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| − | | |
| − | A symbol 'foo' that refers to a non-@property-qualified function or function template is rewritten to foo() iff it does not occur in one of the following contexts:
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| − | | |
| − | 1. Function call position ( foo(args) )
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| − | 2. Address-taken position ( &foo )
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| − | 3. Instantiation position ( foo!args )
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| − | 4. Template argument position ( tmpl!(args1, foo, args2) )
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| − | | |
| − | (Note that redundant parentheses are assumed to be dropped in the parsing stage.)
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| − | | |
| − | == @property: basic design ==
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| − | | |
| − | A global @property function may have one or two arguments. (those are intended to be used UFCS-style only)
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| − | A member @property function may have zero or one argument. (additionally to the optional implicit this pointer)
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| − | | |
| − | In the first cases, the function is a @property getter. In the second cases, it is a @property setter.
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| − | | |
| − | (For templated functions, this restriction is checked after template instantiation as is usual.)
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| − | | |
| − | The following rewrite is assumed to be always applied on the entire AST WLOG: (this gets rid of an annoying border case)
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| − | (exprs, b) (op)= exp is rewritten to (exprs, b (op)= exp)
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| − | | |
| − | The UFCS case is not discussed in detail, but it is straightforward.
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| − | | |
| − | The following __traits is introduced:
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| − | __traits(propertyAccessors, propertySymbol)
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| − | | |
| − | This expression behaves like the propertySymbol, except that it is not subject to any of the rewrite rules below.
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| − | | |
| − | A symbol 'prop' that refers to a @property-qualified function or function template is rewritten to __traits(propertyAccessors, prop)() iff it does not occur in one of the following contexts:
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| − | | |
| − | 1. Assignment position: prop = exp
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| − | -- In this case, there are two possibilities:
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| − | -- - typeof(__traits(propertyAccessors, prop)(exp)) is void => The whole expression is rewritten to (__traits(propertyAccessors, prop)(exp), prop)
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| − | -- - otherwise, the whole expression is rewritten to __traits(propertyAccessors, prop)(exp)
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| − | | |
| − | == @property: possible extensions ==
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| − | | |
| − | Everything below should be appended to the proposal if the consensus is to support simple "semantic rewrites":
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| − | --------------------------------------------------------------------------------------------------------------
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| − | | |
| − | 2. OpAssign position: prop op= exp
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| − | -- In this case, the whole expression is rewritten to {auto val=prop; val op= exp; return prop=val; }(), where 'val' is choosen such that it does not occur free in exp or prop.
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| − | | |
| − | 3. Prefix increment/decrement position: ++prop or --prop
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| − | -- In this case, the whole expression is rewritten to prop+=1 or prop-=1 respectively.
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| − | | |
| − | 4. Suffix increment/decrement position: prop++ and prop--
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| − | -- In this case, the usual rewrites for prop++ and prop-- are applied before prop is rewritten.
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| − | | |
| − | Everything below should be appended to the proposal if the consensus is to support full "semantic rewrites":
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| − | ------------------------------------------------------------------------------------------------------------
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| − | | |
| − | The following cases only apply if typeof(prop) can be successfully evaluated to a value type. i.e. a type that is not a class reference, pointer, slice, function pointer, or delegate.
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| − | | |
| − | 5. IndexAssign and OpIndexOpAssign: prop[indexes] (op)= exp
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| − | -- In this case, the whole expression is rewritten to {auto val=prop; val[indexes] (op)= exp; return prop=val; }(), where 'val' is choosen such that it does not occur free in exp or prop.
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| − |
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| − | 6. Field access position: (context) prop.ident (context), if the context corresponds to any one context from 1 to 6, but not only 6
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| − | -- In this case, the expression is rewritten to {auto val1=prop, val2=(context) val1.ident (context); prop=val1; return val2 }(), where 'val1'/'val2' are choosen such that they does not occur free in exp or prop or in each other.
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| − | (This last rule is formulated in a somewhat informal way and could be stated more rigorously if the need arises.)
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| − | | |
| − | == Applicability of "semantic rewrites" to operator overloading ==
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| − | | |
| − | The "semantic rewrite" strategy would probably need to be employed for operator overloading as well:
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| − | | |
| − | S s; // s is a struct, s.x is a field
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| − | ++s[i].x;
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| − | | |
| − | the last expression would be rewritten according to almost identical rules to
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| − | | |
| − | {auto val1=s[i], val2=++val1.x; s[i]=val1; return val2; }()
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| − | | |
| − | == A note on implementation ==
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| − | | |
| − | The compiler is of course free to use more efficient strategies that lead to the same semantics.
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| − | Error messages should ideally not expose the rewritten code.
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