I think in this case it's a reasonable approach to writing the `dpll` function because that function doesn't actually care about the specifics of the types. Th

I think in this case it's a reasonable approach to writing the `dpll` function because that function doesn't actually care about the specifics of the types. This is a nice example of encapsulation and implementation hiding, a concept familiar to object-oriented and functional programmers alike. He could have easily gotten it to compile by stubbing things out as follows: data Clauses = Clauses data Symbols = Symbols data Model = Model isTrueIn = undefined isFalseIn = undefined findPureSymbol = undefined minus = undefined including = undefined findUnitClause = undefined I think it's more a question of personal preference than right or wrong.