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Going back to at least the late 1990s there have been people wishing for the integration of restricted monads into Haskell in a friendly way.
For example, without restricted monads you can't make an efficient monad out of Set, Map or probability distributions. Here's a SO question from a few years ago where someone else ran afoul of this problem.
There are various workarounds that people have come up with, including:
Creating a new type class for every possible restriction.
Using Template Haskell.
Using Constraint Kinds.
None of these approaches seem to be "canonical" however. I found a comment from Don Stewart on this blog post, in 2007, where he intimated that we were "quite close" to having restricted monads with Indexed types.
What is the current status? Is there now a 'canonical' way to do restricted monads? Or we are still living with workarounds?
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In functional programming, a monad is a software design pattern with a structure that combines program fragments (functions) and wraps their return values in a type with additional computation.
A monad is an algebraic structure in category theory, and in Haskell it is used to describe computations as sequences of steps, and to handle side effects such as state and IO. Monads are abstract, and they have many useful concrete instances. Monads provide a way to structure a program.
Monads are not considered pure or impure. They're totally unrelated concepts. Your title is kind of like asking how verbs are considered delicious. "Monad" refers to a particular pattern of composition that can be implemented on types with certain higher-kinded type constructors.
Using Maybe is a good way to deal with errors or exceptional cases without resorting to drastic measures such as error . The Maybe type is also a monad. It is a simple kind of error monad, where all errors are represented by Nothing . A richer error monad can be built using the Either type.
There's a recent paper by Anders Persson, Emil Axelsson, and Josef Svenningson that shows a way to encode restricted monads. I've forgotten the details, but I remember it was a nice paper.
Persson, A. ; Axelsson, E. ; Svenningsson, J. (2011). Generic monadic constructs for embedded languages. IFL 2011, the 23rd Symposium on Implementation and Application of Functional Languages.
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Actually it is possible to obtain an efficient Set monad as a regular monad, without any restrictions. In two distinct ways. The following article explains both:
http://okmij.org/ftp/Haskell/set-monad.html
The article also points out that restricted monads are actually quite restricted and preclude many monadic idioms. I conjecture that the implementation methods are general and any restricted monad can be turned into the usual one, without losing efficiency. So, it may seem that we don't need restricted monads at all.
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