In imperative languages and also in

In imperative languages and also in some functional languages expression evaluation is strict and immediate. If you write x=y+z; in C , for example , you are telling the computer to perform a computation and put a value into the memory called 'x' right now! ( whenever the code is encountered ). In Haskell , by contrast , evaluation is lazy expressions of f and g in the example show a simple form of the difference.

While a function like g is somewhat silly , since y is just not used , functions with pattern matching or guards will very often use particular arguments only in certain circumstances. If some arguments have certain properties , those or other arguments might not be necessary for a given computation. In such cases , the needless computations are not performed. Furthermore , when lists are expressed in computational ways ( list comprehensions and enumeration ellipsis form ) , only as many list elements as are actually utilized are actually calculated