[swift-dev] AST – Understanding @lvalue

[Greg Titus]

An lvalue is something that can be assigned to / changed. The terminology comes from assignment statements: in “foo = bar”, “foo” is a left-value (lvalue) and “bar" is a right-value (rvalue). The difference is that the lvalue is a named location that can take on a new value and an rvalue is just a plain value.

See: https://en.wikipedia.org/wiki/Value_(computer_science)#lrvalue <https://en.wikipedia.org/wiki/Value_(computer_science)#lrvalue>

In Swift’s AST, you’ll see @lvalue as the type coming from a declaration reference (declref) for var’s and for inout parameters inside a method implementation. You’ll see function application require @lvalue types for inout parameters (most of which will be the implicit first parameters of ‘mutating’ functions - the thing to be mutated). A load converts an expression from an lvalue to being an rvalue, and a store takes an lvalue and a new rvalue to be put there.

Hope this helps!
	- Greg

> On May 24, 2016, at 9:48 AM, Tim Bodeit via swift-dev <swift-dev at swift.org> wrote:
> 
> I couldn’t find any documentation about lvalues in Swift. Can anybody give me a short explanation of what exactly an lvalue is?
> 
> In C++ all variables, including those marked with the const modifier seem to be lvalues. It seems to me, that this is not the case for let constants in Swift.
> 
> I compared the output of swiftc -dump-ast for 
> let i = 42
> let j = i+1
> and
> var i = 42
> let j = i+1
> 
> Where i was respectively accessed as:
> 
> (declref_expr type='Int' location=lvaluetest.swift:2:9 range=[lvaluetest.swift:2:9 - line:2:9] decl=lvaluetest.(file).i at lvaluetest.swift <mailto:i at lvaluetest.swift>:1:5 direct_to_storage specialized=no)
> 
> (load_expr implicit type='Int' location=lvaluetest.swift:2:9 range=[lvaluetest.swift:2:9 - line:2:9]
>   (declref_expr type='@lvalue Int' accessKind=read location=lvaluetest.swift:2:9 range=[lvaluetest.swift:2:9 - line:2:9] decl=lvaluetest.(file).i at lvaluetest.swift <mailto:i at lvaluetest.swift>:1:5 direct_to_storage specialized=no))
> 
> Is my assumption correct, that let constants are not lvalues?
> 
> Cheers,
> 
> Tim
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