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Manual: add extended description of warning 57

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octachron 2016-02-12 17:44:52 +01:00 committed by Gabriel Scherer
parent 6c9729d21a
commit 81447489d3
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manual/manual/cmds/comp.etex
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@ -497,6 +497,7 @@ that are currently defined are ignored. The warnings are as follows.
\begin{options}
\input{warnings-help.tex}
\end{options}
Some warnings are described in more detail in section~\ref{s:comp-warnings}.
The default setting is "-w +a-4-6-7-9-27-29-32..39-41..42-44-45".
It is displayed by "ocamlc -help".
@ -748,3 +749,49 @@ command line, and possibly the "-custom" option.
\end{options}
\section{Warning reference} \label{s:comp-warnings}
This section describes and explains in detail some warnings:
\begin{options}
\item[Warning 57: ambiguous variables in or-patterns]
The semantics of or-patterns in OCaml is specified with
a left-to-right bias: a value \var{v} matches the pattern \var{p} "|" \var{q}
if it matches \var{p} or \var{q}, but if it matches both,
the environment captured by the match is the environment captured by
\var{p}, never the one captured by \var{q}.
While this property is generally intuitive, there is at least one specific
case where a different semantics might be expected.
Consider a pattern followed by a when-guard:
"|"~\var{p}~"when"~\var{g}~"->"~\var{e}, for example:
\begin{verbatim}
| ((Const x, _) | (_, Const x)) when is_neutral x -> branch
\end{verbatim}
The semantics is clear:
match the scrutinee against the pattern, if it matches, test the guard,
and if the guard passes, take the branch.
In particular, consider the input "(Const"~\var{a}", Const"~\var{b}")", where
\var{a} fails the test "is_neutral"~\var{a}, while \var{b} passes the test
"is_neutral"~\var{b}. With the left-to-right semantics, the clause above is
{\em not} taken by its input: matching "(Const"~\var{a}", Const"~\var{b}")"
against the or-pattern succeeds in the left branch, it returns the
environment \var{x}~"->"~\var{a}, and then the guard
"is_neutral"~\var{a} is tested and fails, the branch is not taken.
However, another semantics may be considered more natural here:
any pair that has one side passing the test will take the branch. With this
semantics the previous code fragment would be equivalent to
\begin{verbatim}
| (Const x, _) when is_neutral x -> branch
| (_, Const x) when is_neutral x -> branch
\end{verbatim}
This is {\em not} the semantics adopted by OCaml.
Warning 57 is dedicated to these confusing cases where the
specified left-to-right semantics is not equivalent to a non-deterministic
semantics (any branch can be taken) relatively to a specific guard.
More precisely, it warns when guard uses ``ambiguous'' variables, that are bound
to different parts of the scrutinees by different sides of a or-pattern.
\end{options}