资料来源 : Free On-Line Dictionary of Computing

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
        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 list}s 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)