Nice work David! I say put it under :info. This feature is very useful for beginners and we can't expect them to know about some other info command, what it does, and how it would help them. I too would probably not remember to use this if it's hidden somewhere else. Cheers, Adam On Mon, Oct 19, 2015 at 9:32 PM, Doug Burke wrote:

Or

When t is []: find :: (a -> Bool) -> [a] -> Maybe a

if this really is for the newcomer!

Doug

On Mon, Oct 19, 2015 at 3:11 PM Richard Eisenberg wrote:

...

I would avoid the `(t ~ []) =>` prefix -- it might be confusing to the newcomers this feature is meant to serve. How about

With t := []: find :: (a -> Bool) -> [a] -> Maybe a

?

Richard

On Oct 19, 2015, at 12:47 PM, David Kraeutmann wrote:

...

Hi,

I'm working on a GHC feature [1] that allows users to view specializations of functions like in (name subject to change)

----

...

:binfo find find :: Foldable t => (a -> Bool) -> t a -> Maybe a Specialisations: t ~ [] => find :: (a -> Bool) -> [a] -> Maybe a

...

:binfo first first :: Arrow a => a b c -> a (b, d) (c, d) Specialisations: a ~ (->) => first :: (b -> c) -> (b, d) -> (c, d)

---

Now there are two things left to decide on: * What command should the specializations be visible under? We can either put it under :info or make a special command :sinfo or :binfo that displays only the expression's type and specialisations like in the example above?

* What specializations do we want to display? goldfire suggested using the default list, which has the added benefit of being customizable by e.g. instructors, so adding a default Maybe (in conjunction with [2]) gives the desired behavior and not too many specializations.

Thanks for your time! David

---- Links: [1] : https://ghc.haskell.org/trac/ghc/ticket/10972 [2] : https://ghc.haskell.org/trac/ghc/ticket/10971

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On Oct 20, 2015, at 9:21 AM, Tillmann Rendel wrote:

Looks very helpful. Three ideas:

(1) I think this would be easier to read if the specialized type signature would come first. Maybe like this:

find :: Foldable t => (a -> Bool) -> t a -> Maybe a

Specializations:

find :: (a -> Bool) -> [a] -> Maybe a -- if f ~ [] find :: (a -> Bool) -> Either b a -> Maybe a -- if f ~ Either b

Yes -- I like this more, too.

(2) Can this be made to work for expressions, too?

$ :type-so-that-i-get-it find even find even :: (Foldable t, Integral a) => t a -> Maybe a

Specializations:

find even :: [Integer] -> Maybe Integer -- if t ~ [], a ~ Integer

I like this, too. And this example also brings up the possibility of a combinatorial number of specializations that could be printed. We'd want to limit it, I think.

(3) Could the specializations also be shown in haddock documentation?

Yes yes yes please please please. Richard

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