"Found hole"

Hello! What is a "hole"? This program fails to compile: main = _exit 0 I get this error message: ex.hs:1:8: Found hole ‘_exit’ with type: t Where: ‘t’ is a rigid type variable bound by the inferred type of main :: t at ex.hs:1:1 Relevant bindings include main :: t (bound at ex.hs:1:1) In the expression: _exit In an equation for ‘main’: main = _exit When I replace "_exit" with "foo", it produces a "not in scope" error, as expected. What is special about "_exit"? It doesn't occur in the Haskell Hierarchical Libraries. Bye Volker