StateT's catchError discards changes to state

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Issue 类型
文档
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documentation

调研方向

The issue identifies the StateT documentation as the place to explain this behavior, but names no documentation file or test. Start by locating the StateT documentation and reviewing its existing description of catchError. Done means the docs explain that state changes made before an error are discarded in this transformer stack.

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描述

type: documentation

StateT's catchError doesn't take into account that the state can change in the call to is first parameter:

instance monadErrorStateT :: MonadError e m => MonadError e (StateT s m) where
  catchError (StateT m) h =
    StateT \s -> catchError (m s) (\e -> case h e of StateT f -> f s)

StateT's function m COULD change state and if it does, that state change is lost, i.e. it's neither captured nor is it passed to f.

This is ONLY noticeable when you have a Stack where StateT is ABOVE ExceptT otherwise StateT's catchError will NOT be called.

After many tests, I've come to the conclusion that this CANNOT be fixed. Here was my initial attempt to fix it:

instance monadErrorStateT :: MonadError e m => MonadError e (StateT s m) where
  catchError (StateT m) h =
    StateT \s -> m s >>= \t@(Tuple _ s') -> catchError (pure t) (\e -> case h e of StateT f -> f s')

The problem is that ExceptT is BELOW StateT in the Stack. That means that m s >>= ... will Short-circuit, which is NOT the desired behavior here.

I cannot see any way to capture the changes made to the state by the first call that won't Short-circuit.

So, the only question that remains is how to communicate this information in the docs. The only place that makes sense in the StateT docs.

主要语言
PureScript
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