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Comment by chongli | Hacker News Reader

I think the approach taken by the author is simply the wrong one. Writing a huge amount of code without any clue what the types are going to be is just silly.

Haskell makes it very easy for you if you work with it in a more interactive fashion, asking it to infer types for you as you go along. One of the best features for doing this is Typed Holes [0].

[0] https://wiki.haskell.org/GHC/Typed_holes

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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.

Yeah I always use the type checker to help me refactor too.

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