Modernise diff and main code

Applies the same code style improvements
from previous commit, but also:

- Change the interface of `fenv.parse.diff`
- Abort on error prints execution and maintain status
- fenv.main propagates execution’s return code

functions/fenv.main.fish

@@ -22,25 +22,29 @@
 
 
 function fenv.main
-  set PROGRAM $argv
-  set DIVIDER (fenv.parse.divider)
-  set OLD_ENV (bash -c 'env')
+  set program $argv
+  set divider (fenv.parse.divider)
+  set previous_env (bash -c 'env')
 
-  if not set PROGRAM_EXECUTION (bash -c "$PROGRAM; echo; echo '$DIVIDER'; env")
-    return
-  end
+  set program_execution (bash -c "$program && (echo; echo '$divider'; env)" ^&1)
+  set program_status $status
 
-  if not contains -- (fenv.parse.divider) $PROGRAM_EXECUTION
-    echo "Foreign environment found and error! Aborting to avoid damage!"
-    return
+  if not contains -- "$divider" $program_execution
+    printf '%s\n' $program_execution
+    return $program_status
   end
 
-  set PROGRAM_OUTPUT (fenv.parse.before $PROGRAM_EXECUTION)
-  set NEW_ENV (fenv.parse.after $PROGRAM_EXECUTION)
+  set program_output (fenv.parse.before $program_execution)
+
+  if test $program_status != 0
+    printf "%s\n" $program_output
+    return $program_status
+  end
 
-  set ENVIRONMENT_DIFF (fenv.parse.diff "$OLD_ENV $DIVIDER $NEW_ENV")
+  set new_env (fenv.parse.after $program_execution)
 
-  fenv.apply $ENVIRONMENT_DIFF
+  fenv.apply (fenv.parse.diff "$previous_env" "$new_env")
 
-  printf "%s\n" $PROGRAM_OUTPUT
+  printf "%s\n" $program_output
+  return $program_status
 end

functions/fenv.parse.diff.fish

@@ -21,14 +21,13 @@
 # SOFTWARE.
 
 
-function fenv.parse.diff
-    set SEGMENTED_ENVIRONMENT (echo $argv | tr ' ' '\n')
-    set OLD_ENV (fenv.parse.before $SEGMENTED_ENVIRONMENT)
-    set NEW_ENV (fenv.parse.after $SEGMENTED_ENVIRONMENT)
+function fenv.parse.diff -a env_before env_after
+  set -l before (echo $env_before | tr ' ' '\n')
+  set -l after (echo $env_after | tr ' ' '\n')
 
-    for environment in $NEW_ENV
-        if not contains -- $environment $OLD_ENV
-            echo $environment
-        end
+  for environment in $after
+    if not contains -- "$environment" $before
+      echo $environment
     end
+  end
 end