I have only made money out of Haskell a few times. Most of my use of Haskell is in the academia; My MPhil thesis was on observational equivalence between lazy programs in presence of selective strictness. Haskell is a member of this family of languages where everything is lazy but there are primitives like seq which can enforce strictness. With colleagues, we carved a new induction principle using which we proved three interesting observational equivalences between Haskell programs.