Add introduction

effectful-core/src/Effectful.hs

@@ -49,5 +49,44 @@ import Effectful.Internal.Monad
 
 -- $intro
 --
--- TODO
+-- Haskell is one of the few programming languages that distinguishes between
+-- pure functions and functions that might perform side effects. For example, a
+-- function
+--
+-- @
+-- f :: 'Int' -> 'String'
+-- @
+--
+-- can't perform side effects at all, but a function
+--
+-- @
+-- f :: 'Int' -> 'IO' 'String'
+-- @
+--
+-- can perform any side effect. This "all or nothing" approach isn't very
+-- satisfactory though, because the vast majority of time we would like to
+-- signify that a function can perform /some/ side effects, e.g. only be able to
+-- log messages and have a read only access to the logged in user.
+--
+-- This library provides support for expressing exactly that, using its 'Eff'
+-- monad:
+--
+-- @
+-- f :: (Log ':>' es, 'Effectful.Reader.Static.Reader' User ':>' es) => 'Int' -> 'Eff' es 'String'
+-- @
+--
+-- It implements support for extensible effects with both dynamic and static
+-- dispatch (for more information about each type consult the documentation in
+-- "Effectful.Dispatch.Dynamic" and "Effectful.Dispatch.Static").
+--
+-- It provides:
+--
+-- - The 'Eff' monad that tracks effects at the type level.
+--
+-- - A set of predefined, basic effects such as 'Effectful.Error.Static.Error',
+--   'Effectful.Reader.Static.Reader', 'Effectful.State.Static.Local.State' and
+--   'Effectful.Writer.Static.Local.Writer'.
+--
+-- - Utilities for defining new effects and interpreting them, possibly in terms
+--   of already existing ones.
 --