[Haskell-cafe] Parse do block problem

joeltt joel at harpsoft.com
Tue Dec 22 00:43:25 EST 2009

Yes that if block did get indented incorrectly in the posting. However, I
like how the guards work for this, and as soon as I used your
recommendation, it started working. Great, my first Haskell program, albeit
with a little help, I appreciate it.

David Menendez-2 wrote:
> On Mon, Dec 21, 2009 at 11:14 PM, joeltt <joel at harpsoft.com> wrote:
>> 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 <dave at zednenem.com>
> <http://www.eyrie.org/~zednenem/>
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