On Fri, Jul 26, 2013 at 5:08 PM, Micah Cowan <micah@cowan.name> wrote:

I was wondering if there was a way to do it in "pure" Haskell (i.e., no
GHC pragmas required), and also the specific reason for why the above
example doesn't work without the pragma (I think it's just that in
general a -> b is not syntactically allowed for type specifiers within
instance declarations)?

The error message you get without the pragma tells you exactly what's wrong, and that's not it.

Standard Haskell is *very* conservative about what it allows in an instance declaration; you may not have literal types, nor may you repeat a type variable, only things of the form (Type var1 var2 ...) are permitted. (The (String -> String) is not syntactically a problem; it's read as ((->) String String) which would conform *if* it didn't use literal types. You can verify this by rephrasing it in prefix form --- note the error message uses the infix form even if you phrase it as a prefix!) This is widely seen as unnecessarily restrictive.

brandon s allbery kf8nh sine nomine associates

allbery.b@gmail.com ballbery@sinenomine.net

unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net