Re: [Haskell-cafe] YAWQ (Yet Another Wash Question)

Jeremy Shaw writes:

I am making a wild guess because I do not have all the information in front of me right now but would this work ?

... do x <- if cond

...

then textInputField ... else return ()

... Let me make another guess, probably an even wilder one, ... You have to return a common type for both branches of the if. In the code snippet from above, you either get back a handle to a text input field or () --- so the types won't fit. To unify the types of both branches, I guess you have to introduce an new "wrapper" data type that *mabye* holds a handle of an input field *m* with return value *a* and validity flag *x*. data MaybeInputField m a x = MaybeInputField (Maybe (m a x)) It is important that all "input field type constructors" take two type arguments---one for the return type, and another one for the validity. Otherwise you won't be able to pass the input field to a submit button. (MaybeInputField TexxtInputField) for example fits into that scheme. With that in place, you may be able to write something like the following ... ... do mH <- if cond then do h <- textInputField ... return (MaybeInputField (Just h)) else return (MaybeInputField Nothing) -Matthias

On Feb 24, 2005 08:42 AM, John Goerzen wrote:

...

Thanks for everyone that's helped me out with my Wash questions. I have one more.

I have a textInputField that I only want to display on a form in certain cituations. I can't figure out how to make this work. For instance:

... do if cond then x <- textInputField ... else () ... submit ...

Well, two problems there... first, the scope of the x doesn't reach to the submit. Second, there's a type issue. So I thought maybe I could figure out something like this:

... do x <- if cond then textInputField ... else ()

still stuck with the typing problem.

So I thought maybe something like this...

... do let x = if cond then Just $ textInputField ... else Nothing

Though I suspect this won't work either, since the textInputField won't actually be processed at the right place in the program.

I then thought maybe I could kludge by this with a hidden field, but I can't find one of those, either.

Ideas, anyone?

Thanks, John

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