Syntax error in instance head (variable expected)

Question

I'm reading the book "The Haskell Road to Logic, Math and Programming" and I download their code from the second chapter(http://homepages.cwi.nl/~jve/HR/), but when I try to compile the code, it doesn't work.

The error message is:

TAMO.hs:85 - Syntax error in instance head (variable expected)

What variable is expected? I've never used Haskell before and have no idea what the compiler(Hugs) want me to do to fix it.

class TF p where 
  valid :: p -> Bool
  lequiv :: p -> p -> Bool
instance TF Bool
  where
  valid  = id
  lequiv f g = f == g

instance TF p => TF (Bool -> p)
  where
  valid f = valid (f True) && valid (f False)
  lequiv f g = (f True) `lequiv` (g True)
           && (f False) `lequiv` (g False)

score 3 · Accepted Answer · edited May 23 '17 at 12:31

3

It compiled for me, but I had to fix the indentation (lequiv needed to be at the same level as valid), and add the FlexibleInstances language extension using

{-# LANGUAGE FlexibleInstances #-}

(I am using GHC, I am not sure what the equivalent for HUGS is)

Additional info:

The last answer in Instance Show for function shows you how to use FlexibleInstances in Hugs.

edited May 23 '17 at 12:31

Community

1
1

answered Mar 23 '16 at 01:59

jamshidh

12,002
17
31

Sorry, but I meant the number "85" was the line 85. – jvrn3 Mar 23 '16 at 02:05
1

Thank you, I'll be using GHC from now on – jvrn3 Mar 23 '16 at 02:21

Syntax error in instance head (variable expected)

1 Answers1