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Manual: add extended description of warning 57 #468

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Manual: add extended description of warning 57 #468

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45d6ded
Manual: add extended description of warning 57
Octachron Feb 12, 2016
3323470
Manual: fix warning 57 description
Octachron Feb 12, 2016
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47 changes: 47 additions & 0 deletions manual/manual/cmds/comp.etex
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Expand Up @@ -482,6 +482,7 @@ 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".
Expand Down Expand Up @@ -733,3 +734,49 @@ 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.

The warning 57 is dedicated to warning about 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}
20 changes: 10 additions & 10 deletions testsuite/tests/typing-warnings/ambiguous_guarded_disjunction.ml.reference
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Expand Up @@ -18,7 +18,7 @@ then just above there should be *no* warning text.
# Characters 46-71:
| ((Val x, _) | (_, Val x)) when x < 0 -> ()
^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 57: Ambiguous guarded pattern, variable x may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable x may match different or-pattern arguments (see manual section 8.5)
val ambiguous_typical_example : expr * expr -> unit = <fun>
# Note that an Assert_failure is expected just below.
# Exception: Assert_failure ("//toplevel//", 23, 6).
Expand All @@ -33,19 +33,19 @@ val ambiguous_typical_example : expr * expr -> unit = <fun>
# Characters 33-72:
| (`B (x, _, Some y) | `B (x, Some y, _)) when y -> ignore x
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 57: Ambiguous guarded pattern, variable y may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable y may match different or-pattern arguments (see manual section 8.5)
val ambiguous__y : [> `B of 'a * bool option * bool option ] -> unit = <fun>
# * * * * * * * * val not_ambiguous__rhs_not_protected :
[> `B of 'a * bool option * bool option ] -> unit = <fun>
# Characters 35-74:
| (`B (x, _, Some y) | `B (x, Some y, _)) when x < y -> ()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 57: Ambiguous guarded pattern, variable y may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable y may match different or-pattern arguments (see manual section 8.5)
val ambiguous__x_y : [> `B of 'a * 'a option * 'a option ] -> unit = <fun>
# Characters 37-76:
| (`B (x, z, Some y) | `B (x, Some y, z)) when x < y || Some x = z -> ()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 57: Ambiguous guarded pattern, variables y,z may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variables y,z may match different or-pattern arguments (see manual section 8.5)
val ambiguous__x_y_z : [> `B of 'a * 'a option * 'a option ] -> unit = <fun>
# val not_ambiguous__disjoint_in_depth :
[> `A of [> `B of bool | `C of bool ] ] -> unit = <fun>
Expand All @@ -54,7 +54,7 @@ val ambiguous__x_y_z : [> `B of 'a * 'a option * 'a option ] -> unit = <fun>
# Characters 40-76:
| `A (`B (Some x, _) | `B (_, Some x)) when x -> ()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 57: Ambiguous guarded pattern, variable x may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable x may match different or-pattern arguments (see manual section 8.5)
val ambiguous__in_depth :
[> `A of [> `B of bool option * bool option ] ] -> unit = <fun>
# val not_ambiguous__several_orpats :
Expand All @@ -66,7 +66,7 @@ val ambiguous__in_depth :
# Characters 43-140:
....`A ((`B (Some x, _) | `B (_, Some x)),
(`C (Some y, Some _, _) | `C (Some y, _, Some _))).................
Warning 57: Ambiguous guarded pattern, variable x may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable x may match different or-pattern arguments (see manual section 8.5)
val ambiguous__first_orpat :
[> `A of
[> `B of 'a option * 'a option ] *
Expand All @@ -75,7 +75,7 @@ val ambiguous__first_orpat :
# Characters 44-141:
....`A ((`B (Some x, Some _, _) | `B (Some x, _, Some _)),
(`C (Some y, _) | `C (_, Some y))).................
Warning 57: Ambiguous guarded pattern, variable y may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable y may match different or-pattern arguments (see manual section 8.5)
val ambiguous__second_orpat :
[> `A of
[> `B of 'a option * 'b option * 'c option ] *
Expand All @@ -97,13 +97,13 @@ val ambiguous__second_orpat :
# Characters 40-73:
..X (Z x,Y (y,0))
| X (Z y,Y (x,_))
Warning 57: Ambiguous guarded pattern, variables x,y may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variables x,y may match different or-pattern arguments (see manual section 8.5)
val ambiguous__amoi : amoi -> int = <fun>
# module type S = sig val b : bool end
# Characters 56-101:
....(module M:S),_,(1,_)
| _,(module M:S),(_,1)...................
Warning 57: Ambiguous guarded pattern, variable M may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variable M may match different or-pattern arguments (see manual section 8.5)
val ambiguous__module_variable :
(module S) * (module S) * (int * int) -> bool -> int = <fun>
# val not_ambiguous__module_variable :
Expand All @@ -123,7 +123,7 @@ It will remain exhaustive when constructors are added to type t.
Characters 55-107:
| A (x as z,(0 as y))|A (0 as y as z,x)|B (x,(y as z)) when g x (y+z) -> 1
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Warning 57: Ambiguous guarded pattern, variables x,y may match different or-pattern arguments
Warning 57: Ambiguous guarded pattern, variables x,y may match different or-pattern arguments (see manual section 8.5)
val ambiguous_xy_but_not_ambiguous_z : (int -> int -> bool) -> t -> int =
<fun>
#
2 changes: 1 addition & 1 deletion utils/warnings.ml
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Expand Up @@ -453,7 +453,7 @@ let message = function
"variables " ^ String.concat "," vars in
Printf.sprintf
"Ambiguous guarded pattern, %s may match different or-pattern \
arguments"
arguments (see manual section 8.5)"
msg
| No_cmx_file name ->
Printf.sprintf
Expand Down