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subtyping of tuples 

git-svn-id: http://caml.inria.fr/svn/ocamldoc/trunk@10180 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
master
Jacques Garrigue 2008-11-12 12:21:39 +00:00
parent dc91bf2a12
commit 4a07cb6024
1 changed files with 7 additions and 6 deletions
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13
manual/manual/refman/expr.etex
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@ -776,11 +776,12 @@ which may be an extensible type, if every $typ_i$ is a subtype of $typ'_i$.
\subsubsection*{Variance}
Other types do not introduce new subtyping, but they may propagate the
subtyping of their arguments. For instance, $typ_1 " * " ... " * "
typ_n$ is a subtype of $typ'_1 " * " ... " * " typ'_n$ when every
$typ_i$ is a subtype of $typ'_i$. For function types, the relation is
more subtle: $typ_1$ "->" $typ_2$ is a subtype of $typ'_1$ "->"
$typ'_2$ if $typ_1$ is a supertype of $typ'_1$ and $typ_2$ is a
subtyping of their arguments. For instance, $typ_1 " * " typ_2$ is a
subtype of $typ'_1 " * " typ'_2$ when $typ_1$ and $typ_2$ are
respectively subtypes of $typ'_1$ and $typ'_2$.
For function types, the relation is more subtle:
$typ_1$ "->" $typ_2$ is a subtype of $typ'_1$ "->" $typ'_2$
if $typ_1$ is a supertype of $typ'_1$ and $typ_2$ is a
subtype of $typ'_2$. For this reason, function types are covariant in
their second argument (like tuples), but contravariant in their first
argument. Mutable types, like "array" or "ref" are neither covariant
@ -792,7 +793,7 @@ parameter is covariant if it has only covariant occurences,
contravariant if it has only contravariant occurences,
variance-free if it has no occurences, and nonvariant otherwise.
A variance-free parameter may change freely through subtyping, it does
not to be a subtype or a supertype.
not have to be a subtype or a supertype.
%
For abstract and private types, the variance must be given explicitly,
otherwise the default is nonvariant. This is also the case for