[Haskell-cafe] do/if/then/else confusion

[David Carter]

Another newbie question, but I can't seem to find any answers on the web...

Can someone tell me what's wrong with this?

import qualified System.Posix.Directory as PD

readdirAll :: PD.DirStream - IO [String]
readdirAll d =
  do dir - PD.readDirStream d
 if dir == 
   then return []
   else rest - readdirAll d
return (dir:rest)

Compiling with GHC 6.6.1 gives me the not-very-useful message Parse 
error in pattern, pointing to the i of if. I've tried all kinds of 
alternative indentations (including indenting the else more), 
bracketings etc, but nothing helps.


Thanks

David



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Re: [Haskell-cafe] do/if/then/else confusion

[Brandon S. Allbery KF8NH]

On Nov 1, 2007, at 13:47 , David Carter wrote:


   else rest - readdirAll d


You need another do here to use the - syntax.

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Re: [Haskell-cafe] do/if/then/else confusion

[Jules Bean]

David Carter wrote:

readdirAll :: PD.DirStream - IO [String]
readdirAll d =
  do dir - PD.readDirStream d
 if dir == 
   then return []
   else rest - readdirAll d
return (dir:rest)

Compiling with GHC 6.6.1 gives me the not-very-useful message Parse 
error in pattern, pointing to the i of if. I've tried all kinds of 


You've worked out the answer, and that's fine. You also said later in 
the thread that the error message isn't very good. (Which I agree with)


Perhaps it might help to understand why you do, in fact, get that 
strange message. It might illuminate why it's quite so hard to get good 
error messages in haskell :-( [*]


When you reach the i in if, ghc knows that you are in a do 
expression. A do expression is a sequence of statements separated by 
semicolons (or layout, standing in for semicolons). When we decode the 
layout, everything indented further is part of the same statement. So 
in particular, your code parses as


do {
dir - PD.readDirStream d ;
if dir ==  then return [] else rest - readdirAll d return (dir:rest);
}

The first statement in the do block is fine.

The second is the problem. Now statements are of two basic forms. They 
either have binders, or not. They are either


foo - expr

or simply

expr

Since the second line contains the magic -, it must be the former. So 
what you have is 'if dir ==  then return [] else rest' being the 
'foo', i.e. the bit before the -.


Now what is *supposed* to come before - is a pattern (normally a 
variable name, which is a simple case of a pattern). if is a keyword 
which is not permitted to be a variable name or part of a pattern.


Hence parse error in pattern.

Not sure if that is at all edifying, but it was an interesting exercise 
to write it...


Jules

* there are two reasons it's really hard to write a haskell compiler 
with good error messages. (1) the layout rule. (2) the use of exotic 
type system tricks like type classes to achieve elegant syntax. I don't 
mean to say that the developers of haskell compilers shouldn't keep 
trying though!




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