Now, since no good deed goes unpunished, let me toss another question (or two) at you. Here is some code that only works with the do construct, and I can't see why it is necessary. This occurs regardless of whether anything else in the program uses the do construct, so it isn't one of those cases where something that is called is monadic and thus the caller needs to be monadic as well. The fragment is:
displaySolutions (x:xs) = do displayOneSolution x displaySolutions xs
Why is the "do" necessary for the second pattern? If I leave it out, the compiler complains that I'm applying "displayOneSolution" to too many arguments, namely "x displaySolutions xs".
Without the do, Haskell doesn't recognize it as an attempt to use monadic syntax so it reads it as
displaySolutions (x:xs) = displayOneSolution x displaySolutions xs
That is, displayOneSolution applied to 3 arguments.
Parentheses don't help; then it just complains that I'm applying "(displayOneSolution x)" to too many arguments, namely "displaySolutions xs". Other placements of parentheses change the error message but do not eliminate the problem.
The problem is that you have to use monadic bind instead of the implicit apply. If you don't use monad syntax, you have to write >> or >>= explicitly:
displaySolutions (x:xs) = displayOneSolution x >> displaySolutions xs
One last question. What is the syntax to use in an IO environment to do nothing? For example, I have a function of type "IO ()", with some patterns, and I want to add another pattern that accepts an empty list and, essentially, does nothing with it. Do I use "return ()"?
In _all monads_ the syntax is 'return ()' -- Alastair Reid alastair@reid-consulting-uk.ltd.uk Reid Consulting (UK) Limited http://www.reid-consulting-uk.ltd.uk/alastair/