StateT's catchError discards changes to state

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  • ¿Algo no está bien?
#129 6 comentarios 0 reacciones 0 asignados Ver en GitHub

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Evaluación

Dificultad
1/5
Tiempo estimado
Menos de una hora
Aptitud para principiantes
28/100
Tipo de issue
Documentación
Claridad
Bien especificado
Estado de actividad
Estancado
Área
documentation

Línea de trabajo

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.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

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.

Lenguaje dominante
PureScript
Estrellas
72
Forks
43
Merge medio
2 d 23 h
PR fusionados (30 d)
1

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