Martin DeMello said:
I'm having a lot of trouble mixing file io and wxhaskell's varCreate/Get/Set functions. I have functions
readWords :: String -> IO WordMap wordGrid :: WordMap -> Layout
And within my GUI code, the following compiles (ignores the variable, basically):
words <- varCreate (do {w <- readWords "words"; return w})
I'm not familiar with wxHaskell, but I don't think wxHaskell is your problem here. It looks like you are confusing yourself with overuse of "do" notation, and perhaps a lack of understanding of the monad laws. Whenever you see this: v <- expr return v You can replace it with just "expr". Your code above reduces to: words <- varCreate (readWords "words") So the variable you are creating has type (IO WordMap), when you probably wanted it to have type WordMap. By itself, this compiles, because IO actions are values as good as any other. The problem only manifests when you combine it with code that tries to read the variable as if it had type WordMap. This is probably what you wanted (untested): w <- readWords "words" words <- varCreate w Now, the variable has type WordMap.
wGrid <- do w <- readWords "words" return $ wordGrid w
but I can't get the following (noncompiling code, but it shows what I'm trying to do) working:
wGrid <- do w <- varGet words return $ wordGrid w
With the above change, this should now work, though it might be clearer to write this as: wGrid <- liftM wordGrid $ varGet words
Could someone give me a minimal example of reading in a list of words from a file, binding a wxHaskell variable to the list, and then mapping some GUI code over it?
(Also, I'm making the base assumption that varSet and varGet are wxHaskell's analogue of the State monad - should I be looking at using StateT instead?)
The wxHaskell Var type is not quite like the State monad. It is a type of mutable variable in the IO monad, much like the IORef type. Try to avoid gratuitous use of mutable variables, as they can weaken your ability to reason about your code. I'm not convinced you really need one here. What problem are you trying to solve by using a mutable variable?