Ambiguous type variable
I'm trying to use catch (...) (\e -> putStrLn $ show e) However, I get an error Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved. This goes away if I change the code to catch (...) (\e -> putStrLn $ show (e::IOException)) A couple of things I don't understand here: - The signature for catch begins "Exception e", and exception it "class (Typeable e, Show e) => Exception e". So why isn't show automatically available? - Why does the new code work at all? e is Exception, not IOException. What would happen if it caught a different Exception?
On Thu, Aug 17, 2017 at 07:55:28AM +0000, Jonathon Delgado wrote:
I'm trying to use catch (...) (\e -> putStrLn $ show e) However, I get an error Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved. This goes away if I change the code to catch (...) (\e -> putStrLn $ show (e::IOException))
A couple of things I don't understand here: - The signature for catch begins "Exception e", and exception it "class (Typeable e, Show e) => Exception e". So why isn't show automatically available? - Why does the new code work at all? e is Exception, not IOException. What would happen if it caught a different Exception?
IOException is a concrete type while Exception is a typeclass. In the end, the compiler needs the former, the latter not being enough. The code works as any other class-based function would someFunction :: Monoid a -> [a] -> a -- ^-- in the end `Monoid a` will become something concrete, like -- a String, a Sum, etc. Does that make sense?
On Thu, Aug 17, 2017 at 10:55 AM, Jonathon Delgado
I'm trying to use catch (...) (\e -> putStrLn $ show e) However, I get an error Ambiguous type variable ‘a0’ arising from a use of ‘show’ prevents the constraint ‘(Show a0)’ from being solved. This goes away if I change the code to catch (...) (\e -> putStrLn $ show (e::IOException))
A couple of things I don't understand here: - The signature for catch begins "Exception e", and exception it "class (Typeable e, Show e) => Exception e". So why isn't show automatically available? - Why does the new code work at all? e is Exception, not IOException. What would happen if it caught a different Exception?
In my experience, the most common thing people need is "catch all synchronous exceptions." The word "synchronous" there is the source of a lot of confusion, and relates to a complicated topic of asynchronous exceptions. My recommendation is: don't worry about that right now, use the safe-exceptions package, and switch from catch to catchAny. More details on the package are available at: https://haskell-lang.org/library/safe-exceptions
participants (3)
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Francesco Ariis -
Jonathon Delgado -
Michael Snoyman