G'day all. On Mon, May 12, 2003 at 10:04:05AM -0700, Iavor Diatchki wrote:
The declared type here refers to the signature of "bar". It says that bar should work for any "t", but the functional dependency forces "t" to be Bool.
Actually, the functional dependency forces t to be _something_, but until you see the instance, you don't know what that something is.
In a sense the original program is wrong.
...and in a similar sense the final program is right. In the code which I cut this example down from, it's actually a more desirable type declaration, because rather than "Bool", the true return type of the function in question is a complex type generated using typeclass meta-programming which clients probably don't want to know about.
The way I think of the declaration is as follows: "if there was an instance for 'Foo Char t' (for all t) I could make you a 't' as declared in the body of the function". Now since there is no instance for 'Foo Char t' you would never be able to use "bar", and in that sense the program is wrong (well, perhaps useless).
True, in the original program, there is "no instance". However, this is just a cut-down illustration. To be useful, it is sufficient for the relevant instance of the Foo typeclass to be in scope for whoever calls "bar". Cheers, Andrew Bromage