I believe the feature "typed holes" first came about around ghc 7.8. I'm
not sure if there ever was a pragma for it because it got everyone so
excited it was just made to default on from its inception.
On Mon, Jan 19, 2015 at 9:29 AM, Elias Diem
Hi there
I'm referring to a post from Stefan Höck and will quote from there:
http://article.gmane.org/gmane.comp.lang.haskell.beginners/14198
<quote> Now, load this into GHCi or compile with GHC. If it compiles, you're on the right track. Now, you want to implement it using a fold (try both, foldl and foldr):
last5 :: [a] -> Maybe a last5 xs = foldr _ _ xs
The underscores are 'type holes'. This tells the compiler to give you some information about what is supposed to be placed at the two positions. For the moment, we are only interested in the types of the things that go there. The compiler will tell you, that the hole at the first position is of type
a -> Maybe a -> Maybe a </quote>
How does the compiler tell me this? I didn't find any flags for GHC to turn this on. What do I miss?
-- Greetings Elias
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