[Haskell-cafe] A small error

[Andrew Coppin]

David House wrote:
> On 19/05/07, Andrew Coppin <andrewcoppin at btinternet.com> wrote:
>> Um... why doesn't this work?
>>
>> my_fn x = do
>>   if x < 0
>>     then do
>>       let y = 5
>>       return ()
>>     else do
>>       let y = 8
>>       return ()
>>
>>   print y
>>
>> (Error: "y" is not in scope.)
>
> let-blocks only scope over the do-block they're contained in.

Ouch!

> Here y
> is out of scope once you leave the inner do-block. The solution is to
> do something like the following:
>
> do
>  let y = if x < 0 then 5 else 8
>  print y

Well, in my case the thing in each do block is an actual IO action. (It 
asks the user some questions, rather than just generating a constant.)

How about this?

  do
    y <- if x < 0
       then do ...
       else do ...