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- `succ` and `pred` are unused.
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- The default definitions of `enum`\* would make more sense if `toEnum` and `fromEnum` used `Integer` instead of `Int`.
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## The Functor class
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Proposal:
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- Add instances for `((->) a)`, `((,) a)` and `Either a`.
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## The Monad class
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- Monads are really functors, but programmers can't always assume that `fmap` is defined for them, and so must use `liftM` instead.
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Similarly, code parameterized by `Functor` cannot be used with monads.
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- The `fail` method was added to the class in Haskell 98 to implement pattern match failure in `do` expressions.
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However the assumption that errors are strings can be problematic (e.g. cf the `Error` class in [ Control.Monad.Error](http://www.haskell.org/ghc/docs/6.4-latest/html/libraries/mtl/Control-Monad-Error.html), or for internationalization).
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Proposals:
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- Add instance for `((->) a)`.
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- Make `Functor` a superclass of `Monad`.
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This would impose an extra burden on those who just want to define a `Monad`.
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- Make `join` a method of `Monad`, interdefined with `(>>=)`.
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