wta: Guard against too many transitions

When the user requests more transitions than the maximal number for the given
automaton, the current generator runs into an endless loop trying to find new
unused transitions. We now detect this case early and throw an error.

src/random-wta/Generator.hs

@@ -112,6 +112,11 @@ genTransitions' numTransitions = do
   let n = numStates wtaSpec
       m = IndexedTransition.maxIndex wtaSpec
 
+  -- FIXME Handle this case better. A nicer error output or something along this
+  -- line.
+  when (fromIntegral numTransitions >= m) $ do
+    error $ "genTransitions: More transitions than possible requested"
+
   transitions <- lift $ map (IndexedTransition.fromIndex wtaSpec) <$> uniqueTransitions numTransitions m
   weightedTransitions <- (traverse.traverse.traverse) (const genMonoidValue) transitions
 

src/random-wta/IndexedTransition.hs

 {-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE DerivingStrategies #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
 
 module IndexedTransition (IndexedTransition(..), maxIndex, fromIndex) where
 
@@ -10,6 +12,7 @@ import Data.Tuple
 import Types
 
 newtype IndexedTransition = Index Integer
+  deriving newtype (Num, Eq, Ord)
   deriving (Show)
 
 maxIndex :: WTASpec m -> IndexedTransition