[m-rev.] diff: io.get_environment_var on java

[Peter Wang]

Branches: main

Make io.get_environment_var really look up environment variables on Java.
Previously it looked up "system properties", probably because Java versions
1.2 through 1.4 didn't support looking up of environment variables, due to
the (unfortunately not life-threatening) medical condition called HUA.

library/io.m:
        Make io.get_environment_var really look up environment variables
        on Java.

        Make io.set_environment_var throw an exception on Java as it
        isn't supported.

diff --git a/library/io.m b/library/io.m
index c7f3c3e..5d518a1 100644
--- a/library/io.m
+++ b/library/io.m
@@ -1275,6 +1275,8 @@
     % is the value to be assigned to that variable. Will throw an exception
     % if the system runs out of environment space.
     %
+    % Note: this predicate is not supported on Java.
+    %
 :- pred io.set_environment_var(string::in, string::in, io::di, io::uo) is det.
 
 %-----------------------------------------------------------------------------%
@@ -9835,22 +9837,8 @@ command_line_argument(_, "") :-
     io.getenv(Var::in, Value::out),
     [will_not_call_mercury, tabled_for_io],
 "
-    // Note that this approach only works for environmental variables that
-    // are recognized by Java and hava a special format.
-    // (eg os.name, user.timezone etc)
-    // To add custom environmental variables, they must be set at the
-    // command line with java's -D option
-    // (ie java -DENV_VARIABLE_NAME=$ENV_VARIABLE_NAME ClassName)
-    // XXX Perhaps a better approach would be to determine the OS at
-    // runtime and then Runtime.exec() the equivalent of 'env'?
-
-    try {
-        Value = java.lang.System.getProperty(Var);
-        succeeded = (Value != null);
-    } catch (java.lang.Exception e) {
-        Value = null;
-        succeeded = false;
-    }
+    Value = System.getenv(Var);
+    succeeded = (Value != null);
 ").
 
 :- pragma foreign_proc("Erlang",
@@ -9902,14 +9890,11 @@ io.setenv(Var, Value) :-
     io.setenv(Var::in, Value::in),
     [will_not_call_mercury, tabled_for_io],
 "
-    // XXX see io.getenv/2
+    // Java does not provide a method to set environment variables, only a way
+    // to modify the environment when creating a new process.
 
-    try {
-        Value = java.lang.System.setProperty(Var, Value);
-        succeeded = true;
-    } catch (java.lang.Exception e) {
-        succeeded = false;
-    }
+    // Avoid warning: Var, Value
+    succeeded = false;
 ").
 
 :- pragma foreign_proc("Java",

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