An Introduction to Functional Programming Through Lambda Calculus

- 161 - : For objects which do not have printable value representations SML will still display the types. In particular, functionvalues are displayed as:fn : Types are described by type expressions. We will see how these are constructed as we discuss specific types.9.3. Basic types - booleans, integers and stringsThe type expression for a basic type is the type’s identifier.The boolean type has identifier:booland values:true falseFor example:- true;> true : boolThe integer type has identifier:intwith positive and negative integer values, for example:- 42;> 42 : int- ˜84˜84 : intNote the use of ˜ as the negative sign.The string type has identifier:stringString values are character sequences within- "Is this a string?";> "Is this a string?" : string9.4. ListsIn SML, unlike LISP and our approach to λ calculus, a list must contain elements of the same type and end with theempty list. Thus, lists cannot be used to represent records with different type fields.Lists are written as , separated element sequences within [ and ]. For example: