An Introduction to Functional Programming Through Lambda Calculus

- 166 -For example:- "Happy"ˆ" birthday!";> "Happy birthday!" : stringThe operator:sizereturns the size of a string:- size;> fn : string -> intFor example:- size "hello";> 5 : intStandard functions for turning strings into string lists are discussed below.9.10. List standard functionsIn SML, list operations apply to lists of any types. In SML, an unknown type is denoted by a single letter namepreceded by a prime - ’, for example:’a ’b ’cThus, we can refer to a list of arbitrary typed objects as having type:’a listIn SML, lists are accessed by the head and tail operators:hd tlThe head operator returns the head object with type:’afrom an arbitrary typed list. Thus, hd is of type:- hd;> fn : (’a list) -> ’aFor example:- hd [1,2,3,4,5];> 1 : intSimilarly, the tail operator returns the tail with type:’a listfrom an arbitrary type list. Thus, tl is of type: