Haskell : using guards after where

Question

Is it possible to use guards to define a function after a where is Haskell?

This works fine :

myMax a b = a + b - myMin a b
    where myMin a b = if a < b then a else b

But this

myMax a b = a + b - myMin a b
    where myMin a b
    | a < b      = a
    | otherwise  = b

will throw the following error message in ghci :

parse error (possibly incorrect indentation or mismatched brackets)

on the line corresponding to | a < b = a

Just as well, this will work :

myMax a b = 
  let myMin a b = if a < b then a else b  
  in  a + b - myMin a b

But this won't :

myMax a b = 
  let myMin a b 
    | a < b = b
    | otherwise = a 
  in  a + b - myMin a b

My guess would be that it's because using guards does not actually define a variable, even though it is required by the where and let/in structures? I am very new to Haskell so any explanation is greatly welcome!

bheklilr · Accepted Answer · 2014-05-02 20:26:27Z

12

You have to indent the guards past the declaration of your function:

myMax a b = a + b - myMin a b
    where myMin x y  -- Also, use different variable names here to avoid conflicts
            | x < y     = x
            | otherwise = y

Or as

myMax a b =
    let myMin x y
            | x < y     = x
            | otherwise = y
    in a + b - myMin a b

If you're using tabs for indentations, I would highly recommend to using spaces, they're less ambiguous with Haskell. You can use tabs, but I see a lot of people run into parse errors because of it.

answered May 2, 2014 at 20:26

bheklilr

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2 Comments

Chuck Over a year ago

The thing about using tabs for indentation is that they're fine on their own, but mixing tabs and spaces leads to misleading situations (e.g. three spaces visually looks like much less indentation than two tabs but is actually more). And since your files will almost certainly include spaces, it's tabs that have to go.

Jeremy List Over a year ago

I usually put let and where on their own lines unless the entire scope is going to fit easily on a single line.

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