[Haskell-beginners] Ambiguous type variable

Jonathon Delgado voldermort at hotmail.com
Thu Aug 17 14:18:41 UTC 2017

Thank you very much, that does clarify things!

Francesco Ariis  wrote:

That's correct! Indeed ghc is not complaining about a lack of 
instances, as it would with, say 

    λ> putStrLn 5 
    -- • No instance for (Num String) arising from etc etc. 

but about the *ambiguity* of type variable `e`.  How does `catch` know 
_which_ exception to deal with if you don't specify the concrete type? 

    prova = catch (print $ div (error "hey bby") 0) 
                  (\e -> print "ouch" >> 
                         print (e :: ErrorCall)) 

    -- I want to deal with arithmetic errors here and not in `prova` 
    palla = catch prova 
                  (\e -> print "baaah" >> 
                         print (e :: ArithException)) 

If I switch ArithException and ErrorCall the behaviour of invoking `palla` 

Having a catch-all `catch` is possible by using (e :: SomeException); 
if you don't care about `e` and just want to do an action regardless, you 
are probably better off with `onException`. 

Ask more if needed! 

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