An Introduction to Functional Programming Through Lambda Calculus

- 172 -9.18. Tuple selectionTuple elements are selected by defining functions with appropriate bound variable tuples. For example, to select thename, department and ’phone number from a telephone directory entry tuple:- fun tname (n:(string * string),d:string,p:int) = n;> val tname = fn : ((string * string) * string * int) -> (string * string)- fun tdept (n:(string * string),d:string,p:int) = d;> val tdept = fn : ((string * string) * string * int) -> string- fun tno (n:(string * string),d:string,p:int) = p;> val tno = fn : ((string * string) * string * int) -> intTo avoid writing out bound variables which are not used in the function body, SML provides the wild card variable:_which behaves like a nameless variable of arbitrary type. For example, we could rewrite the above examples as:- fun tname (n:(string * string),_,_) = n;> val tname = fn : ((string * string) * ’a * ’b) -> (string * string)- tname (("Anna","Able"),"Accounts",123);> ("Anna","Able") : (string * string)- fun tdept (_,d:string,_) = d;> val tdept = fn : (’a * string *’b) -> string- tdept (("Anna","Able"),"Accounts",123);> "Accounts" : string- fun tno (_,_,p:int) = p;> val tno = fn : (’a * ’b * int) -> int- tno (("Anna","Able"),"Accounts",123);> 123 : intNote that SML uses ’a and ’b to stand for possibly distinct unknown types.For nested tuple selection, nested bound variable tuples are used. For example, to select the forename and surnamefrom a telephone directory entry:- fun fname ((f:string,_),_,_) = f;> val fname = fn : ((string * ’a) * ’b * ’c) -> string- fname (("Anna","Able"),"Accounts",123);> "Anna" : string- fun sname ((_,s:string),_,_) = s;> val fname = fn : ((’a * string) * ’b * ’c) -> string- sname (("Anna","Able"),"Accounts",123);> "Able" : string