Question

I defined the following datatype which shall provide 2 functions:

data Problem = Prob { result :: State -> Action -> State
                    , stepCost :: State -> Action -> Integer
                    }

How can I instantiate this type? I tried this:

let mcp = Prob { result _ _ = (False, (1,1)), stepCost _ _ = 1 } in True

however I get:
ERROR – Syntax error in expression (unexpected `_’)

I managed to instantiate a similar datatype with the exception that its members take only 1 argument:

data Lala = La  { omg :: State
                , gee :: Integer
                }

let mcp = La { omg = (False, (1,1)), gee = 1 } in True

returns True.

But the point is that I want to instantiate the datatype Problem with its specific result and stepcost function. Of course, these functions depend on their arguments. So how can I pass them some arguments?

Answer 1

You need to instantiate it with a lambda expression; you can’t treat it as a name that takes parameters directly.

let mcp = Prob { result = \_ _ -> (False, (1,1)), stepCost = \_ _ -> 1 } in True