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Make it more explicit that Null doesn't extend anything but Object.

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Fix dart.sty
Address new comments on https://codereview.chromium.org/2555343004/

R=eernst@google.com

Review-Url: https://codereview.chromium.org/2641873003 .
This commit is contained in:
Lasse R.H. Nielsen 2017-01-20 09:03:43 +01:00
parent e8cf6b43a3
commit fcff3fe4e7
1 changed files with 10 additions and 3 deletions
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13
docs/language/dartLangSpec.tex
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@ -2631,7 +2631,7 @@ The reserved word \NULL{} evaluates to the {\em null object}.
\LMHash{} \LMHash{}
The null object is the sole instance of the built-in class \code{Null}. Attempting to instantiate \code{Null} causes a run-time error. It is a compile-time error for a class to extend, mix in or implement \code{Null}. The null object is the sole instance of the built-in class \code{Null}. Attempting to instantiate \code{Null} causes a run-time error. It is a compile-time error for a class to extend, mix in or implement \code{Null}.
The \code{Null} class declares no methods except those also declared by \code{Object}. The \code{Null} class extends the \code{Object} class and declares no methods except those also declared by \code{Object}.
\LMHash{} \LMHash{}
The static type of \NULL{} is the \code{Null} type. The static type of \NULL{} is the \code{Null} type.
@ -7404,7 +7404,7 @@ A type $T$ is more specific than a type $S$, written $T << S$, if one of the fo
\begin{itemize} \begin{itemize}
\item $T$ is $S$. \item $T$ is $S$.
\item $T$ is $\bot$. \item $T$ is $\bot$.
\item $T$ is \NULL{} and $S$ is not $\bot$. \item $T$ is \code{Null} and $S$ is not $\bot$.
\item $S$ is \DYNAMIC{}. \item $S$ is \DYNAMIC{}.
\item $S$ is a direct supertype of $T$. \item $S$ is a direct supertype of $T$.
\item $T$ is a type parameter and $S$ is the upper bound of $T$. \item $T$ is a type parameter and $S$ is the upper bound of $T$.
@ -7426,13 +7426,20 @@ $List <: List<String>$ and $List<int> <: List$, but $List<int>$ is not a subtype
Although $<:$ is not a partial order on types, it does contain a partial order, namely $<<$. This means that, barring raw types, intuition about classical subtype rules does apply. Although $<:$ is not a partial order on types, it does contain a partial order, namely $<<$. This means that, barring raw types, intuition about classical subtype rules does apply.
} }
\commentary{
The \code{Null} type is more specific than all non-$\bot$ types, even though
it doesn't actually extend or implement those types.
The other types are effectively treated as if they are {\em nullable},
which makes \NULL{} assignable to them.
}
\LMHash{} \LMHash{}
$S$ is a supertype of $T$, written $S :> T$, iff $T$ is a subtype of $S$. $S$ is a supertype of $T$, written $S :> T$, iff $T$ is a subtype of $S$.
\commentary{The supertypes of an interface are its direct supertypes and their supertypes. } \commentary{The supertypes of an interface are its direct supertypes and their supertypes. }
\LMHash{} \LMHash{}
An interface type $T$ may be assigned to a type $S$, written $T \Longleftrightarrow S$, iff either $T <: S$, $S <: T$, or either $T$ or $S$ is the \code{Null} type. An interface type $T$ may be assigned to a type $S$, written $T \Longleftrightarrow S$, iff either $T <: S$, $S <: T$.
\rationale{This rule may surprise readers accustomed to conventional typechecking. The intent of the $\Longleftrightarrow$ relation is not to ensure that an assignment is correct. Instead, it aims to only flag assignments that are almost certain to be erroneous, without precluding assignments that may work. \rationale{This rule may surprise readers accustomed to conventional typechecking. The intent of the $\Longleftrightarrow$ relation is not to ensure that an assignment is correct. Instead, it aims to only flag assignments that are almost certain to be erroneous, without precluding assignments that may work.