An Introduction to Functional Programming Through Lambda Calculus

- 102 -LIST_EQUAL (TAIL [2,3]) (TAIL [2,4]) -> ... ->LIST_EQUAL [3] [4] -> ... ->{ EQUAL (HEAD [3]) (HEAD [4])) -> ... ->EQUAL 3 4 -> ... ->FALSE}FALSE6.10. StringsStrings are the basis of text processing in many languages. Some provide strings as a distinct type, for exampleBASIC and ML. Others base strings on arrays of characters, for example Pascal and C. Here we will introduce stringsas linear lists of characters.We can define a test for stringness:ISSTRING [] = TRUEISSTRING (H::T) = (ISCHAR H) AND (ISSTRING T)For example:ISSTRING [’a’,’p’,’e’] -> ... ->(ISCHAR ’a’) AND(ISSTRING [’p’,’e’]) -> ... ->(ISCHAR ’a’) AND((ISCHAR ’p’) AND(ISSTRING [’e’])) -> ... ->(ISCHAR ’a’) AND((ISCHAR ’p’) AND((ISCHAR ’e’) AND(ISSTRING []))) -> ... ->(ISCHAR ’a’) AND((ISCHAR ’p’) AND((ISCHAR ’e’) ANDTRUE) -> ... ->TRUEIn our notation, this function is:rec ISSTRING S =IF ISNIL STHEN TRUEELSE AND (ISCHAR (HEAD S)) (ISSTRING (TAIL S))We will represent a string as a sequence of characters within"Here is a string!"