On Mon, Dec 21, 2009 at 11:14 PM, joeltt
I'm trying to write my first Haskell program. The below is the first real logic block I've tried to write, unfortunately I get a "The last statement in a 'do' construct must be an expression" error when loading the method. However, the location of this problem isn't very clear. Is there a way to get more detailed parse message from Haskell, or can someone tell me where the problem is (and better "why"?). I don't think I actually need to use a "do" IO/Monad theme here, but its not clear to me either way. This isn't homework, its just for fun...
do_solve_iter guess tried = do let actual = count_occurences guess if guess == actual then putStrLn "ANSWER!!" else if (find (==actual) tried) == Just actual then do putStrLn "NO ANSWER!" putStrLn tried else do putStrLn "ITER" do_solve_iter actual (actual : tried)
Assuming your indentation didn't get lost in transmission, your
problem is the line "if guess == actual", which needs to be at the
same level of indentation as "let actual...". As written, the do-block
is terminating after the let-statement, which isn't permitted.
Also, unless there is more to the function, you don't really need the
outermost do-block at all. You can rewrite it easily as a let...in
expression, or move the definition of actual to a where clause and use
guards to avoid the nested if-expressions.
do_solve_iter guess tried
| guess == actual = putStrLn "ANSWER!!"
| find (==actual) tried == Just actual = do
putStrLn "NO ANSWER"
putStrLn tried
| otherwise = do
putStrLn "ITER"
do_solve_iter actual (actual : tried)
where
actual = count_occurences guess
--
Dave Menendez