[m-rev.] for review: fix bug #226 - incorrect purity of lazy.read_if_val/2

[Julien Fischer]

For review by Paul.

The comments in the bug report mentioned that we were awaiting some changes
to the deep profiler before doing this, but since there is currently
only a single
call to read_if_val/2 in the deep profiler, I assume that these have been done.

I'm not sure that the purity promise in deep_profiler/autopar_types.m is in the
correct spot -- it's currently equivalent to the existing code except
the impurity
is no explicit.

---------------------------------------

Fix bug #226: the library predicate lazy.read_if_val/2 was incorrectly declared
as pure.  The reason for it being impure rather than semipure is to avoid any
goals containing calls to force/1 being reordered with respect to it.

library/lazy.m:
    Make read_if_val/2 as impure.

deep_profiler/autopar_types.m:
     Confrom to the above change.

Julien.

diff --git a/deep_profiler/autopar_types.m b/deep_profiler/autopar_types.m
index 5d101b1..9565b6c 100644
--- a/deep_profiler/autopar_types.m
+++ b/deep_profiler/autopar_types.m
@@ -310,14 +310,16 @@
pard_goal_detail_annon_to_pard_goal_annon(SharedVarsSet, PGD, PG) :-
     assoc_list(var_rep, float)::in, assoc_list(var_rep, float)::out) is det.

 build_var_use_list(Map, Var, !List) :-
-    (
-        map.search(Map, Var, LazyUse),
-        read_if_val(LazyUse, Use)
-    ->
-        UseTime = Use ^ vui_cost_until_use,
-        !:List = [Var - UseTime | !.List]
-    ;
-        true
+    promise_pure (
+        (
+            map.search(Map, Var, LazyUse),
+            impure read_if_val(LazyUse, Use)
+        ->
+            UseTime = Use ^ vui_cost_until_use,
+            !:List = [Var - UseTime | !.List]
+        ;
+            true
+        )
     ).

 %-----------------------------------------------------------------------------%
diff --git a/library/lazy.m b/library/lazy.m
index a42bfce..9979f72 100644
--- a/library/lazy.m
+++ b/library/lazy.m
@@ -69,7 +69,7 @@
     % with force/1 This is useful as it can provide information without
     % incurring (much) cost.
     %
-:- semipure pred read_if_val(lazy(T)::in, T::out) is semidet.
+:- impure pred read_if_val(lazy(T)::in, T::out) is semidet.

     % Test lazy values for equality.
     %
@@ -169,9 +169,7 @@ read_if_val(Lazy, Value) :-
     promise_equivalent_solutions [Mutvar] (
         Lazy = lazy(Mutvar)
     ),
-    promise_semipure (
-        impure get_mutvar(Mutvar, State),
-        State = value(Value)
-    ).
+    impure get_mutvar(Mutvar, State),
+    State = value(Value).

 %-----------------------------------------------------------------------------%