Haskell nested where clauses

Question

I am a beginner coder in haskell, while doing an exercise from the first chapter of this amazing book: http://book.realworldhaskell.org/read/getting-started.html I came across this issue:

-- test comment
main = interact wordCount
 where
     wordCount input = show (ls ++ " " ++ ws ++ " " ++ cs  ++ "\n")
     where
         ls = lines input
         ws = length words input
         cs = length input



wonderbox:ch01 manasapte$ runghc WC < quux.txt
WC.hs:5:9: parse error on input ‘where’

Why can I not nest my wheres ?

Answer 1

Since your second where is attached to the wordCount definition, it needs to be indented more than it. (Although you will still have some other errors afterward.)

Answer 2

Others have already answered. I will just add some more explanation.

Simplifying a bit, the Haskell indentation rule is:

• Some keywords start a block of things (where,let,do,case ... of).
• Find the first word after such keywords and note its indentation. Name the column it occurs the pivot column.
• Start a line exactly on the pivot to define a new entry in the block.
• Start a line after the pivot to continue the entry started in the previous lines.
• Start a line before the pivot to end the block.

Hence,

where
     wordCount input = show (ls ++ " " ++ ws ++ " " ++ cs  ++ "\n")
     where
         ls = lines input
         ws = length words input
         cs = length input

Actually means

where {
     wordCount input = show (ls ++ " " ++ ws ++ " " ++ cs  ++ "\n")
     ;
     where {     -- same column, new entry
         ls = lines input
         ;   -- same column, new entry
         ws = length words input
         ;   -- same column, new entry
         cs = length input
         }
     }

which treats the second where as a separate definition unrelated to wordCount. If we indent it more, it will work:

where {
     wordCount input = show (ls ++ " " ++ ws ++ " " ++ cs  ++ "\n")
       where {     -- after the pivot, same entry
         ls = lines input
         ;
         ws = length words input
         ;
         cs = length input
         }
     }