Editing Merely monadic (section)

== Supplemental operations for monadic types (''non-standard morphisms'') ==

In addition to the two basic ones required by the monadic interface, each monadic type can have its own specific operations e.g. to provide access to and/or enable manipulation of data it implicitly carries which is specific to its nature; cause some specific side-effects; etc. 

=== Running monadic actions ===

Some monadic types provide an extra operation for ''running'' actions, usually if they are ''purely'' defined by the type. This allows an action's output value to be obtained directly in Haskell, in contrast to <code>(>>=)</code>. While some of the predefined (e.g. from the <code>Prelude</code>) monadic types in Haskell have such an operation (e.g. <code>Prelude.maybe</code> or <code>Prelude.either</code>), most do not.

Why this is expressly  '''''not''''' a part of the monadic interface by now should be obvious - impure monadic actions could then be "hidden" inside seemingly-pure definitions, which would end any notion of purity in Haskell. This is why the only general-purpose way to use a monadic action's output value is to direct it to subsequent computations via the reaction which constructs them, through the use of <code>(>>=)</code>.

[[Category:Monad]]
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