I get a Num ambiguous constraint error as well, but it is merely not giving
you the rest of the errors.
Prelude> let a1 = pure (1::Int)
Prelude> let a2 = pure (1::Int)
Prelude> a1 == a2
• Ambiguous type variable ‘f0’ arising from a use of ‘a1’
prevents the constraint ‘(Applicative f0)’ from being solved.
Furthermore you can't compare an IO a with an IO a, so it will still cause
an error. But if you go:
Prelude>let a1 = 1
Prelude>let a2 = 1
Prelude>a1 == a2
True
Then it is totally fine with that.
On Thu, Dec 22, 2016 at 10:38 AM, Imants Cekusins
what is the "default" structure if you don't specify any
similar to:
display::Show a => a -> String display = show
fa::(Applicative f, Num a) => f a fa = pure 1
f and a are *bounded* by Applicative and Num, so to say. No default. Or, it is typed however type is a bit broader than *
In this case it defaults f to IO and a to Int,
does it though?
Prelude> let a1 = pure 1 Prelude> let a2 = pure 1 Prelude> a1 == a2
<interactive>:20:1: error: • Ambiguous type variable ‘a0’ arising from a use of ‘a1’ prevents the constraint ‘(Num a0)’ from being solved.
_______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners