Index: docs/language/dartLangSpec.tex |
diff --git a/docs/language/dartLangSpec.tex b/docs/language/dartLangSpec.tex |
index 1d41cf52ef1fcaf0aed043c928134cf7444dfde9..7df7c9f8de636f4c1344523befab5d689ca1f709 100644 |
--- a/docs/language/dartLangSpec.tex |
+++ b/docs/language/dartLangSpec.tex |
@@ -2631,7 +2631,7 @@ The reserved word \NULL{} evaluates to the {\em null object}. |
\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 \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{} |
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} |
\item $T$ is $S$. |
\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 a direct supertype 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. |
} |
+\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{} |
$S$ 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. } |
\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. |