debugging - Haskell program outputs `<<loop>>`

Question

I wrote a Haskell program that preforms a binary search on a list. At least that's what I thought it does. When I compiled the program with ghc v7.6.3 and ran the program I got the following output:

progname: <<loop>>

What on earth does this output mean? Does it mean I had an infinite loop that ghc optimized away? How am I supposed to debug this?

Answer 1

As several of the comments have said, this is the Haskell RTS detecting an infinite loop at run-time. It cannot always detect such loops, but in simple cases it can.

For example,

x = x + 1

will compile just fine, but provoke an exception at run-time. (Incidentally, this is an exception - in particular, you can catch it if you want. But you probably don't "want".)

So why does GHC even let this compile? Well, because if I replace + with, say, :, then the expression now terminates just fine. (It represents a 1-element circular list.) The compiler can't tell at compile-time what is and is not sensible recursion. The RTS can't always tell at run-time; but when it can tell something's wrong, it'll let you know by throwing an exception at you.