lazy evaluation

lazy evaluation
laziness

   <reduction> An {evaluation strategy} combining {normal order
   evaluation} with updating.  Under normal order evaluation
   (outermost or call-by-name evaluation) an expression is
   evaluated only when its value is needed in order for the
   program to return (the next part of) its result.  Updating
   means that if an expression's value is needed more than once
   (i.e. it is shared), the result of the first evaluation is
   remembered and subsequent requests for it will return the
   remembered value immediately without further evaluation.  This
   is often implemented by graph reduction.  An unevaluated
   expression is represented as a {closure} - a data structure
   containing all the information required to evaluate the
   expression.

   Lazy evaluation is one {evaluation strategy} used to implement
   non-{strict} functions.  Function arguments may be infinite
   data structures (especially lists) of values, the components
   of which are evaluated as needed.

   According to Phil Wadler the term was invented by Jim Morris.

   Opposite: {eager evaluation}.

   A partial kind of lazy evaluation implements lazy data
   structures or especially {lazy lists} where function arguments
   are passed evaluated but the arguments of data constructors
   are not evaluated.

   {Full laziness} is a {program transformation} which aims to
   optimise lazy evaluation by ensuring that all subexpressions
   in a function body which do not depend on the function's
   arguments are only evaluated once.

   (1994-12-14)