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 :: IO ()
main = print $ let a = 10 in
               let a = a in
               a :: Int

I guess this must have something to do with the laziness of Haskell since the same code terminates in OCaml. However, if I wrote the following code instead

main :: IO ()
main = print $ let a = 10 in
               let b = a in
               b :: Int

the code would have no problem terminating at all. I can't get the reason since in the original code, the two a's should be considered as two different variables. I don't know why the naming of them has anything to do with the semantic of the program.

Answer 1

The issue is that, unlike OCaml, let bindings in Haskell are recursive by default. So let x = x in ... is equivalent to OCaml's let rec x = x in ... and is a circular definition.

This is why shadowing variable names in Haskell (ie defining a multiple times) is considered bad style and even has a compiler warning, which you can turn on with the -Wall flag or more specifically -fwarn-name-shadowing.

This default makes more sense in Haskell than OCaml because, thanks to laziness, circular values (rather than just recursive functions) are actually useful. let x = 1:x gives us an infinite list of 1, which we can use just like a normal list.

At the same time, some people don't like this for basically exactly the reason you ran into here: it's possible to introduce unintuitive infinite loops in your code, which makes some errors and typos harder to track down. This is also confusing because, by necessity, <- bindings in do-notation are not recursive by default, which is a bit inconsistent.

Answer 2

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)