They are described at these two links: https://www.haskell.org/haskellwiki/GHC/Typed_holes https://downloads.haskell.org/~ghc/7.8.1-rc1/docs/html/users_guide/typed-hol... Essentially, identifiers that are not otherwise in scope and consist of an underscore or that have a trailing underscore are treated as holes, for which you wish to know which type was inferred. Previously you would need to do something like add a wrong type signature, so that the compiler would complain that the type you gave doesn't match what it inferred. Though I get a different error message: Found hole ‘_exit’ with type: a0 -> t Where: ‘a0’ is an ambiguous type variable ‘t’ is a rigid type variable bound by There really should be a Num constraint as well, but at least in 7.8 it doesn't seem to include those. Volker Wysk-5 wrote
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
_______________________________________________ Glasgow-haskell-users mailing list
Glasgow-haskell-users@
http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
-- View this message in context: http://haskell.1045720.n5.nabble.com/Found-hole-tp5764054p5764057.html Sent from the Haskell - Glasgow-haskell-users mailing list archive at Nabble.com.