RE: Allow ambiguous types (with warning) by default

Sure, I'll open the ticket. It may well turn out that there's a less invasive way to accomplish my ultimate goal (more useful error messages for ambiguous types) without allowing the cascade you're worried about. In foo :: Num a => F a -> G a Something more like the following would be much better than what we currently get: The type variable `a' in the type of `foo :: Num a => F a -> G a' is ambiguous. This means that code attempting to use `foo' will not be able to give it a signature determining `a'. In particular: `a' appears only as an argument to the type families `F' and `G', which may not be injective. Therefore a signature at the use site fixing `F a' and `G a' to particular types will not fix `a' to a particular type. On Dec 7, 2015 8:31 AM, "Simon Peyton Jones" wrote:

| OK, fine. Is there a way to make it an error, but keep checking the | rest of the module? My goal is *get both messages if possible*, within | a module. I'm not tied to any particular mechanism of doing so.

Yes it'd be possible. A bit fiddly, but certainly possible.

Of course, doing so can lead to a cascade of other errors, but in this case you seem to actively want those follow-on errors.

Would you like to open a ticket with a few illustrative examples to motivate your proposal?

Simon

| | On Sun, Dec 6, 2015 at 12:13 AM, Edward Kmett | wrote: | > If you aren't the one writing the code that can't be called you may | > never see the warning. It'll be tucked away in a cabal or stack | build | > log somewhere. | > | > -Edward | > | > On Sun, Dec 6, 2015 at 12:06 AM, David Feuer | wrote: | >> | >> No, I want it to *warn* by default. If I write | >> | >> foo :: something that will fail the ambiguity check bar = something | >> that uses foo in a (necessarily) ambiguous way | >> | >> the current default leads me to do this: | >> | >> 1. Attempt to compile. Get an ambiguity error on foo whose exact | >> cause is hard for me to see. | >> 2. Enable AllowAmbiguousTypes and recompile. Get an error on bar | >> whose exact cause is completely obvious, and that makes it | perfectly | >> clear what I need to do to fix foo. | >> 3. Fix foo, and disable AllowAmbiguousTypes. | >> | >> I'd much rather go with | >> | >> 1. Attempt to compile. Get an ambiguity *warning* on foo whose | exact | >> cause is hard for me to see, but also an error on bar whose exact | >> cause is completely obvious, and that makes it perfectly clear what | I | >> need to do to fix foo. | >> 2. Fix foo. | >> | >> Simple example of how it is currently: | >> | >> > let foo :: Num a => F a; foo = undefined; bar :: Int; bar = foo | >> | >> <interactive>:14:12: | >> Couldn't match expected type ‘F a’ with actual type ‘F a0’ | >> NB: ‘F’ is a type function, and may not be injective | >> The type variable ‘a0’ is ambiguous | >> In the ambiguity check for the type signature for ‘foo’: | >> foo :: forall a. Num a => F a | >> To defer the ambiguity check to use sites, enable | AllowAmbiguousTypes | >> In the type signature for ‘foo’: foo :: Num a => F a | >> | >> Couldn't match what with what? Huh? Where did a0 come from? | >> | >> > :set -XAllowAmbiguousTypes | >> > let foo :: Num a => F a; foo = undefined; bar :: Int; bar = foo | >> | >> <interactive>:16:61: | >> Couldn't match expected type ‘Int’ with actual type ‘F a0’ | >> The type variable ‘a0’ is ambiguous | >> In the expression: foo | >> In an equation for ‘bar’: bar = foo | >> | >> Aha! That's the problem! It doesn't know what a0 is! How can I tell | >> it what a0 is? Oh! I can't, because foo doesn't give me a handle on | it. | >> Guess I have to fix foo. | >> | >> I'd really, really like to get *both* of those messages in one go, | >> with the first one preferably explaining itself a bit better. | >> | >> On Sat, Dec 5, 2015 at 11:51 PM, Edward Kmett | wrote: | >> > So you are saying you want users to write a ton of code that | >> > happens to have signatures that can never be called and only | catch | >> > it when they go to try to actually use it in a concrete situation | >> > much later? | >> > | >> > I don't really show how this would be a better default. | >> > | >> > When and if users see the problem later they have to worry about | if | >> > they are doing something wrong at the definition site or the call | >> > site. With the status quo it complains at the right time that you | >> > aren't going to sit there flailing around trying to fix a call | site | >> > that can never be fixed. | >> > | >> > -Edward | >> > | >> > On Sat, Dec 5, 2015 at 5:38 PM, David Feuer | | >> > wrote: | >> >> | >> >> The ambiguity check produces errors that are quite surprising to | >> >> the uninitiated. When the check is suppressed, the errors at use | >> >> sites are typically much easier to grasp. On the other hand, | >> >> there's obviously a lot of value to catching mistakes as soon as | >> >> possible. Would it be possible to turn that into a warning by | >> >> default? | >> >> | >> >> | >> >> _______________________________________________ | >> >> ghc-devs mailing list | >> >> ghc-devs@haskell.org | >> >> | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fma | >> >> il.haskell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc- | devs&data=01%7 | >> >> | c01%7csimonpj%40064d.mgd.microsoft.com%7c572fb4db692644c7b2d408d2f | >> >> | dfc50bb%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=tzknS1bYbEs2G | >> >> NPvXuu6o1st%2b2VMSTTmRSH1L%2f9BSsk%3d | >> >> | >> > | > | > | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.h | askell.org%2fcgi-bin%2fmailman%2flistinfo%2fghc- | devs%0a&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c572fb4db6926 | 44c7b2d408d2fdfc50bb%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=kYhH | Q00HzSxbbNyqFt2OmHWSpPTyxsbeJPCF1tDzAA0%3d