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 for lazy evaluation
From The Free On-line Dictionary of Computing (30 December 2018) :

  lazy evaluation
      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
     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.

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