An Introduction to Functional Programming Through Lambda Calculus

- 186 -4) Write functions to:i) Extract the leftmost n letters from string s:- left 4 "goodbye";> "good" : stringii) Extract the rightmost n letters from string s:- right 3 "goodbye";> "bye" : stringiii) Extract n letters starting with the l’th letter from string s:- middle 2 5 "goodbye";> "by" : stringiv) Find the position of the first occurence of string s1 in string s2:- find "by" "goodbye";> 5 : int5) The train travelling east from Glasgow Queen’s Street to Edinburgh Waverly passes through Bishopbriggs,Lenzie, Croy, Polmont, Falkirk High, Linlithgow and Edinburgh Haymarket. These stations might berepresented by the data type:datatype station = Queens_Street | Bishopbriggs |Lenzie | Croy |Polmont | Falkirk_High |Linlithgow | Haymarket | Waverly;Write functions which return the station to the east or the west of a given station, for example:6) The data type:- east Croy;> Polmont : station- west Croy;> Lenzie : stationdatatype exp = add of exp * exp |diff of exp * exp |mult of exp * exp |quot of exp * exp |numb of int;might be used to represent strictly bracketed integer arithmetic expressions:so: ::= ( + ) |( - ) |( * ) |( / ) |