Theory

Laziness

• laziness = non-strictness + sharing
• An expression is in weak head normal form (WHNF), if it is either:[^1] - a constructor (eventually applied to arguments): True, Just (Square 42), (:) 1 - a built-in function applied to too few arguments: (+) 2, sqrt - a lambda abstraction: \x -> expression

Graph Reduction

• normal-order reduction

Strictness

• 6.14. Bang patterns and Strict Haskell

• seq

[^1]: Weak head normal form - HaskellWiki