Why this Haskell code never terminates?

Question

I recently wrote some Haskell code and it never terminates. After I carefully examined my code, the problem boiled down to the following code piece

main :: I         


        

Answer 1

The second binding (a = a) shadows the other one. The first example is (almost) exactly equivalent to

main = print $ let xyz = 10 in
               let a = a in
               a :: Int

and I hope it's clear why that one doesn't terminate! You can get GHC to warn you about this by using the -fwarn-name-shadowing flag (or by entering :set -fwarn-name-shadowing in GHCi)