[MonadError example based on the MTL
Samuel Bronson <naesten@gmail.com>**20061024233706] {
addfile ./tests/examples/MonadError.phc
hunk ./tests/examples/MonadError.phc 1
+-- Port of part of Control.Monad.Error to use ATs (and then to phrac)
+
+data Either a b = Left a | Right b;
+
+class Error e where {
+    noMsg :: e;
+    strMsg :: List Char -> e;
+}
+
+instance Error (List Char) where {
+    noMsg = Nil;
+    strMsg s = s;
+}
+
+class Monad m where {
+    return :: a -> m a;
+    (>>=) :: m a -> (a -> m b) -> m b;
+}
+
+(>>) p q = p >>= \_ -> q;
+
+class Monad m => MonadError m where {
+    type MError m;
+    throwError :: MError m -> m a;
+    catchError :: m a -> (MError m -> m a) -> m a;
+}
+    
+instance (Error e) => Monad (Either e) where {
+    return = Right;
+    (>>=) p q = case p of
+                Left e -> Left e;
+                Right x -> q x;;          
+}
+
+instance (Error e) => MonadError (Either e) where {
+    type MError (Either e) = e;
+    throwError = Left;
+    catchError p h = case p of
+                     Left e -> h e;
+                     x -> x;;
+}
+
+main :: Either (List Char) Unit;
+main = return ();
}