Make 'viewAsFieldDescr' a total function.

Fixes haskell#980 by allowing to create config fields from options with optional arguments. Without this change, setting the 'jobs' or 'verbose' fields in the config file results in a fatal error on startup.
mgajda · Aug 16, 2012 · d50b2d3 · d50b2d3
1 parent c247ec7
commit d50b2d3
Showing 1 changed file with 3 additions and 3 deletions.
diff --git a/Cabal/Distribution/Simple/Command.hs b/Cabal/Distribution/Simple/Command.hs
@@ -270,9 +270,9 @@ viewAsFieldDescr (OptionField n dd) = FieldDescr n get set
                                                  Just f -> return (f a)
                                                  _      -> syntaxError line val
                     BoolOpt _ _ _ setV _    -> (`setV` a) `liftM` runP line n parse val
-                    OptArg _ _ _ _readE _ _ -> -- The behaviour in this case is not clear, and it has no use so far,
-                                               -- so we avoid future surprises by not implementing it.
-                                               error "Command.optionToFieldDescr: feature not implemented"
+                    OptArg _ _ _  readE _ _ -> ($ a) `liftM` runE line n readE val
+                                             -- Optional arguments are parsed just like required arguments here;
+                                             -- we don't provide a method to set an OptArg field to the default value.
 
 getChoiceByLongFlag :: OptDescr b -> String -> Maybe (b->b)
 getChoiceByLongFlag (ChoiceOpt alts) val = listToMaybe [ set | (_,(_sf,lf:_), set, _) <- alts