haskell / haskell/error-messages
Better MonadFail
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Description
*from Haskell GameDev discord*
Given a failable `... <- ...` pattern on sum type `Request` with two variants:
Most users probably don't want to implement a `MonadFail` instance for monads defined in a library. The following would be more helpful:
```
• Failable pattern in non-failable do statement
`(HaulRequest (resource, requiredAmount) amount)`
This pattern fails on:
`ConstructionRequest`
• Note: the left pattern of `<-` must succeed on
all variants unless `MonadFail` is defined
• Perhaps you intended to handle each variant with `case`:
`r0 <- get building
case r0 of
HaulRequest ...
ConstructionRequest ...`
• Perhaps you intended to define a
`MonadFail (SystemT World IO)` instance to
handle failable patterns
```
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