Hello,

    I am developing a small game engine (nothing fancy or professional, it is
to test the waters of Haskell). You could say it's a very simple DSL: a
user would import a library and use its tool to make a game.

The problem I encountered is when defining or getting/setting variables for an
instance game. Let me illustrate

    data GameState = GameState [(String, Int)]

This is a very simple and naive gamestate, an association list consisting
in (variableName, itsValue). This would mean restricting values to "Int" only,
but I can easily enough add any kind of value with:

    data GameState = GameState [(String, Dynamic)]

An example helper function signature would look like

    getBool :: String -> Either MyError Bool
    data MyError = NotFound String
                 | TypeMismatch String

The problem with this approach is that I loathe picking up those two kind of
errors (var not found and type mismatch) at runtime. I would like them to be
found at compile time (like "normal" Haskell).

I thought that a possible solution was to declare a datatype with the needed
variables inside:

    data MyGameVar = MyGameVar { health :: Int,
                                 message :: String }
    myvars = MyGameVar 1 "Don't forget your umbrella!"

This would find at compile time typos in variable names and type errors at
compile time, of course. But then I would have to change the GameState to:

    data GameState = GameState MyGameVar

which would be fine if I were developing a *game*, but it is not fine when
you develop a *game engine* (that would mean having to rewrite MyGameVar
every game you made).

So I am left to think if the problem I highlighted is "solvable" or do I
have to make compromises? I guess I am not the first to encounter this
difficulty, so I am asking here.
Any hint or idea is appreciated, thanks!
-F
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