3 expressions types and parametric polymorphism

Since Haskell is statically typed, it can infer from the expression itself what function types we're using. Specifically, you can skip over function:: a -> b and define function directly.

table of contents:

1. 3.1 well formed well typed expressions
2. 3.2 names and operators
3. 3.3 types
4. 3.4 polymorphic types
5. 3.5 selectors, discriminators, deconstructors
6. 3.6 type inference