Am Freitag, 13. Februar 2009 21:08 schrieb Jonathan Cast:
On Fri, 2009-02-13 at 12:15 -0700, John A. De Goes wrote:
On Feb 13, 2009, at 12:07 PM, Jonathan Cast wrote:
Exactly! But if it fails, why on earth should any other use of map in the module succeed?
Because more information is known about other usages of map. Such is the nature of type inference.
No it's not. Type inference -- in Haskell --- means --- by definition! --- looking up the principle type of each sub-term, specializing it based on its use, and then generalizing to find the principle type of the overall term. Adding information can cause type inference to fail, but --- in Haskell as it exists --- it cannot cause type inference to succeed.
I'm not sure about the finer distinctions between type inference and type checking as performed by Haskell implementations when compiling a module, but what about polymorphic recursion, where adding information via a type signature can be necessary to make the compilation succeed? Not what this thread is about, though.
Which is good!
Why is it good? Because using additional information to make type inference succeed would cause an ad hoc and hard to reason about type inference algorithm? Or other reasons?
jcc
Cheers, Daniel