Expanded documentation for classy errors

* Additional documentation describing the use of classy prisms for
  errors

* Added the missing AsExUnitsError (ValidationError era) instance

src/base/lib/Convex/Class.hs

@@ -6,6 +6,7 @@
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE FunctionalDependencies #-}
 {-# LANGUAGE GADTs #-}
+{-# LANGUAGE InstanceSigs #-}
 {-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE TemplateHaskell #-}
@@ -118,6 +119,7 @@ import Control.Exception (
   throwIO,
  )
 import Control.Lens (
+  Prism',
   at,
   set,
   to,
@@ -202,8 +204,25 @@ instance AsExUnitsError (ValidationError era) era where
 
 Then functions throwing ExUnitsError throw like this:
 @
-throwError $ L.review _ExUnitsError $ Phase1Error bla
+fun :: (MonadError e m, AsExUnitsError e era) => m ()
+fun = do
+  ...
+  throwError $ L.review _ExUnitsError $ Phase1Error bla
 @
+and we are not forcing a specific error @e@, but only that @e@ can be created via the prism from an @ExUnitsError@.
+
+It is also possible to have multiple types of errors:
+@
+fun :: (MonadError e m, AsExUnitsError e era, AsValidationError e era, AsSomeOtherError e) => m ()
+fun = do
+  ...
+  throwError $ L.review _ExUnitsError $ Phase1Error bla
+  ...
+  throwError $ L.review _PredicateFailures $ []
+  ...
+  throwError $ L.review _OtherError
+@
+Note that @OtherError@ is "parallel" to @_ExUnitsError@ and not a descendent, i.e. not related.
 
 The whole tree of exceptions can be mapped in this fashion, which increases the interoperability
 between error-throwing functions without the need for explicit 'modifyError' calls.
@@ -229,6 +248,10 @@ instance (C.IsAlonzoBasedEra era) => Show (ValidationError era) where
 
 makeClassyPrisms ''ValidationError
 
+instance AsExUnitsError (ValidationError era) era where
+  _ExUnitsError :: Prism' (ValidationError era) (ExUnitsError era)
+  _ExUnitsError = _VExUnits . _ExUnitsError
+
 -- | Send transactions and resolve tx inputs.
 class (Monad m) => MonadBlockchain era m | m -> era where
   sendTx

src/coin-selection/lib/Convex/CoinSelection.hs

@@ -16,6 +16,9 @@
 {-# LANGUAGE TypeApplications #-}
 {-# LANGUAGE TypeOperators #-}
 {-# LANGUAGE ViewPatterns #-}
+-- NOTE: We have an orphan instance for ScriptWitnessIndex, until (or if) upstream provides such an
+-- instance.
+{-# OPTIONS_GHC -Wno-orphans #-}
 
 -- | Building cardano transactions from tx bodies
 module Convex.CoinSelection (
@@ -227,7 +230,10 @@ bodyError = BodyError . Text.pack . C.docToString . C.prettyError
 
 $(deriveFromJSON defaultOptions{sumEncoding = TaggedObject{tagFieldName = "kind", contentsFieldName = "value"}} ''C.ScriptWitnessIndex)
 
--- | Balancing errors, including the *special* 'BalancingScriptExecutionErr'.
+{- | Balancing errors, including the important 'ScriptExecutionErr'. "Important" in the sense that we
+can write prisms such as 'exactScriptExecutionError' that allow for more fine-grained
+pattern-matching on the on-chain error.
+-}
 data BalancingError era
   = BalancingError Text
   | -- | A single type of balancing error is treated specially: the type with
@@ -243,10 +249,10 @@ data BalancingError era
 
 makeClassyPrisms ''BalancingError
 
-{- | This prism will either match on the exact error using the script witness index and an error
-string, or just on being a script execution error. The latter is necessary, since in non-debug
-mode, there is no error to match on and the prism should not fail in its purpose in non-debug
-mode.
+{- | This prism will match on the exact error using the script witness index and an error
+string if this information is available. It will behave as a script execution error if no debug logs
+are available. The latter is necessary, since in non-debug mode, there is no error to match on and
+the prism should not fail in its purpose in non-debug mode.
 -}
 exactScriptExecutionError :: forall e era. (AsBalancingError e era) => (Int, Text) -> Prism' e [(C.ScriptWitnessIndex, Text, [Text])]
 exactScriptExecutionError (i, s) = prism' tobe frombe
@@ -267,8 +273,12 @@ exactScriptExecutionError (i, s) = prism' tobe frombe
       -- Not the correct error
       | otherwise -> Nothing
 
-{- | Sort *most* balancing errors into 'BalancingError', but script execution errors into their own
-data constructor.
+{- | This convenience function takes @Left@s of type 'C.TxBodyErrorAutoBalance' and throws
+'C.TxBodyScriptErecutionError's as 'ScriptExecutionErr', with extraaction of the error log, if
+availalbe. Other @Left@s are thrown as 'BalancingError's.
+
+This is a convenient bridge from cardano-api errors to errors where we can pattern-match on script
+execution errors and have an error hierarchy.
 -}
 balancingError :: (MonadError (BalancingError era) m) => Either (C.TxBodyErrorAutoBalance era) a -> m a
 balancingError = \case