<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><div>A bit of time away from my keyboard has revealed a natural way to solve this problem and others: be more like Idris.</div><div><br></div><div>Normally, of course, I'm thinking about how Haskell's type system can be more like Idris's. But that's not what I mean here. I want Haskell's interface to be more like Idris's. Imagine this interchange:</div><div><br></div><div>λ> :t ($)</div><div>($) :: (a -> b) -> a -> b</div><div>-- click on the type</div><div>($) :: forall a b. (a -> b) -> a -> b</div><div>-- click on the a</div><div>($) :: forall (a :: *) b. (a -> b) -> a -> b</div><div>-- click on the b</div><div>($) :: forall (a :: *) (b :: *). (a -> b) -> a -> b -- where b's kind has a different color than usual</div><div>-- click on b's kind</div><div>($) :: forall {r :: RuntimeRep} (a :: *) (b :: TYPE r). (a -> b) -> a -> b</div><div>-- mouseover RuntimeRep or TYPE reveals a tooltip</div><div>"($) is representation-polymorphic, meaning that `b` can have an arbitrary runtime representation. Please see http://.... for more details."</div><div><br></div><div>Similarly, classes would render in a special color, allowing you to click on them and choose to instantiate the type at a few in-scope instances of the class at hand, changing Foldable f => f a -> Int to the much simpler [a] -> Int.</div><div><br></div><div>This is not a minor engineering project, but it would reap wonderful rewards, addressing the problems in this thread and more. No more lying (because all lies are clickable), no more fragmented language, no more brakes on development.</div><div><br></div><div>Evidently, Chris already agrees with this proposal: #10073 (<a href="https://ghc.haskell.org/trac/ghc/ticket/10073">https://ghc.haskell.org/trac/ghc/ticket/10073</a>)</div><div><br></div><div>Also see #8809 (<a href="https://ghc.haskell.org/trac/ghc/ticket/8809">https://ghc.haskell.org/trac/ghc/ticket/8809</a>)</div><div><br></div><div>Any volunteers to implement this? :)</div><div><br></div><div>Richard</div><br><div><div>On Feb 5, 2016, at 7:47 PM, Artyom <<a href="mailto:yom@artyom.me">yom@artyom.me</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
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<div style="" class="markdown-here-wrapper" data-md-url="Thunderbird"><p style="margin: 1.2em 0px ! important;">I’ve amended my
suggestion to say basically “this type is a slight lie, here’s a
flag/command to see the true type” – this way we aren’t scaring
people with implementation guts, merely letting them see the
guts for themselves and then think “I don’t care about this”
(which is, I think, exactly what should happen; the worst
scenario here is that the beginner falls into the “I’m an
advanced user, I need all features, I need to know everything,
so I’ll enable the flag” trap – which is why it’s important not
to call it “an advanced type” or mention “if you know what
you’re doing” or anything else like that).</p><p style="margin: 1.2em 0px ! important;">I don’t agree that
levity can be compared to Java’s “class” or “static” – not
because it’s harder to understand, but because it’s much less
widely used; I don’t feel that you need to know about levity in
order to be a good Haskeller. Also, unboxed types don’t imply
knowledge of levity – for instance, I’ve been successfully using
unboxed types for a while, but I only found out about the true
type of <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">($)</code>
by complete accident (I think I queried the kind of <code style="font-size: 0.85em; font-family: Consolas,Inconsolata,Courier,monospace;margin: 0px 0.15em; padding: 0px 0.3em; white-space: pre-wrap; border: 1px solid rgb(234, 234, 234); background-color: rgb(248, 248, 248); border-radius: 3px; display: inline;">-></code>
and then got curious about question marks). Of</p><p style="margin: 1.2em 0px ! important;">On 02/06/2016 03:27 AM,
Mihai Maruseac wrote:</p><div style="margin: 1.2em 0px !important; "><br class="webkit-block-placeholder"></div>
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<pre wrap="">On Fri, Feb 5, 2016 at 7:09 PM, Richard Eisenberg <a class="moz-txt-link-rfc2396E" href="mailto:eir@cis.upenn.edu"><eir@cis.upenn.edu></a> wrote:
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<pre wrap="">Another great question that has come up is about Haddock output (Hackage). I
think Haddock needs to add a facility where library authors can include
specializations of an overly general type. This can be done in commentary,
but it's not as prominent. Such a new feature would address the ($) problem,
as ($) :: forall (a :: *) (b :: *). (a -> b) -> a -> b is a specialization
of its real type. It would also help a great deal with FTP-related
generalizations.
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<pre wrap="">This goes hand in hand with Artyom's suggestion of a warning in GHCi
after showing the simpler type.
I'm thinking of a flag which enables/disables printing the simplest
type with warning (in GHCi) or footnote (or otherwise, in Haddock). We
can have the default behavior of the flag be either printing the
simpler type + extra (warning/footnote) or printing the longer type
and include a reference in our learning materials that beginners and
people confused by the long, complex and real type, can use
--use-simpler-types flag.
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