Quirkiness in Haskell 7.8.3

Question

I have 2 functions in a plain file with nothing else.

--This is FN1
helper :: Int -> Double -> Double
helper n x = (nu/d)
  where
    nu = x^n 
    d = product [1 .. n]

--This is FN2
filterOdd :: [a] -> [a] 
filterOdd ls = fi ls [1..]
  where
    fi [] _ = []
    fi (v:vs) (i:ix) = if even i
                        then v:(fi vs ix) 
                        else fi vs ix

I am using GHC 7.8.3

I know that there is a type error in FN1. I am aware of that. When FN1 is commented and run, GHCi throws no errors. But when FN1 is uncommented and run GHCi show the obvious error in FN1, but also a 2 page long error for FN2.

I don't understand this. There is no explicit relationship between FN1 and FN2. Nothing in FN1 is calling FN2 and vice versa. But why does GHCi show errors for FN2 when FN1 is uncommented?

The exact error is long enough, that I decided to put it on lpaste

Could someone please tell me, what is going on? Mostly likely, I have misunderstood something.

Answer 1

This was GHC Trac ticket #9323, which gave the example

module Foo where

broken :: [Int]
broken = ()

ambiguous :: a -> String
ambiguous _ = show 0

"thomie" closed the ticket after Simon Peyton Jones left this comment in July:

Everything is fine in HEAD. (I don't know which of the many changes between 7.8 and HEAD is responsible.)

I'll add a regression test. But I don't propose to fix 7.8.

Simon

Note: a peculiar but now-entrenched convention in GHC development circles is to refer to the official git master branch as HEAD.