An Introduction to Functional Programming Through Lambda Calculus

- 127 -matches the argument:so:[,,] ==::::::NILI == S == R == We can use the bound variable list:([I,S,R]::T)to match against a stock control list so that [I,S,R] matches the first item and T matches the rest of the list. Forexample, we might find all the items that need to be reordered. If the list is empty then return the empty list.If the listis not empty, if the first item needs to be reordered then add it to those to be reordered in the rest of the list. Otherwisereturn those to be reordered in the rest of the list:rec REORD [] = 0or REORD ([I,S,R]::T) =IF LESS S RTHEN [I,S,R]::(REORD T)ELSE REORD TFor example, we can redefine the telephone directory selector functions as:def ENAME [N,A,P] = Ndef EFORENAME [[F,S],A,P] = Fdef ESURNAME [[F,S],A,P] = Sdef ADDRESS [N,A,P] = Adef PHONE [N,A,P] = PHere, the bound variable list:[N,A,P] == N::A::P::NILmatches the argument list:so:[,,] == ::::::NILN == A == P == Similarly, the bound variable list:[[F,S],A,P] == (F::S::NIL)::A::P::NILmatches the argument list: