On Fri, Feb 11, 2011 at 20:00, Daniel Fischer
... It's because there's no way to determine the type variable a (in either wrong or right).
That's what I thought when I wrote the code at first but then I was surprised to see it working with the Show type-class.
In such cases, under some circumstances, the type variable gets defaulted (the ambiguous contexts all must have the form (C a), at least one numeric class [Num, Integral, Fractional, ...] is involved, all involved classes come from the Prelude or the standard libraries) as specified in the report (section 4.3, iirc).
I'm reading it right now.
... These conditions are not met by your 'right' function, but ghci uses extended default rules, a type variable a with a Show constraint [and no other] gets defaulted to (). But ghci has no rule how to default your Dumb class, so it reports the ambiguous type variable there. [Without extended defaulting rules, ambiguous type variable errors would be too frequent at the ghci prompt.]
God! It seems like I'm reading the small-character lines of a contract :) Seriously, now it makes sense... Thank you. -- Cristiano GPG Key: 4096R/C17E53C6 2010-02-22 Fingerprint = 4575 4FB5 DC8E 7641 D3D8 8EBE DF59 B4E9 C17E 53C6