Hi Folks, In this book [1] the author defines the term "bottom": In order that we can say that, without exception, every syntactically well-formed expression denotes a value, it is convenient to introduce a special symbol (upside down T), pronounced 'bottom', to stand for the undefined value of a particular type. In particular, the value of infinity is the undefined value (bottom) of type Integer, and 1/0 is the undefined value (bottom) of type Float. Hence we can assert that 1/0 = bottom. He defines infinity as this: infinity :: Integer infinity = infinity + 1 The author says this when discussing the Bool datatype: It follows that there are not two but three Boolean values, namely False, True, and bottom. In fact, every datatype declaration introduces an extra anonymous value, the undefined value of the datatype. What is the undefined value (bottom) of type Bool? What is the undefined value (bottom) of type String? If I create my own datatype: data MyBool = F | T What is the undefined value (bottom) of type MyBool? I am not clear why "bottom" is an important concept. Would you explain please? /Roger [1] Introduction to Functional Programming using Haskell by Richard Bird