Re: [Haskell-cafe] A small error

19 May 2007

      David House wrote:
...
On 19/05/07, Andrew Coppin  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 ...