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 <imantc@gmail.com> wrote:

> 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.

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