[Haskell-cafe] Help with TH trick

[David Feuer]

Why the double splice? What about this makes you generate code that when
run will generate code?

On Thu, Nov 3, 2022, 10:10 PM Michael Sloan <mgsloan at gmail.com> wrote:

> Hmm, the documentation does list 5 bullets of why this is a nice way of
> doing things.  But yes, considering this didn't catch on no doubt either
> the docs are poor quality or the thing isn't really something people want.
> FWIW I wrote this many years ago, before deriving via.  I just figured you
> might be interested as it is very similar to your query.
>
> The examples could be better.  One thing to note is that you can list a
> bunch of instances:
>
> $($(derive [d|
>         instance Deriving (Storable X)
>         instance Deriving (Eq X)
>         instance Deriving (Show X)
>         |]))
>
>
> (purely an example, afaik it wouldn't make sense to use TH to generate
> some canonical Eq and Show instances)
>
> Whereas with typical `$(mkStorable [t| Storable X|] $)` style you need
> multiple splices and you need to know the names of these TH functions.
>
> In retrospect this would probably be better without the `Deriving`
> wrapper, I think I needed that to disambiguate from the instantiators,
> which indeed aren't properly documented at all (I don't recall how they
> work, and don't feel like digging into it much further)
>
> On Tue, Oct 25, 2022 at 11:21 PM David Feuer <david.feuer at gmail.com>
> wrote:
>
>> I must admit that looks rather mysterious to me. The documentation I saw
>> doesn't make the benefit of the nested splice terribly obvious.
>>
>> On Tue, Oct 25, 2022, 10:53 PM Michael Sloan <mgsloan at gmail.com> wrote:
>>
>>> You might also be interested in
>>> https://hackage.haskell.org/package/th-utilities-0.2.5.0/docs/TH-Derive.html -
>>> though afaik it didn't catch on.
>>>
>>> The cleverness there is to use nesting splicing, where the inner one
>>> generates code involving something like "runDeriver (Proxy @ Storable X)
>>> ...", where runDeriver is a method of a typeclass.  The instances of this
>>> class implement the code generation that is ultimately used for the output.
>>>
>>> $($(derive [d|
>>>         instance Deriving (Storable X)
>>>         |]))
>>>
>>>
>>> On Sat, Oct 22, 2022, 13:48 David Feuer <david.feuer at gmail.com> wrote:
>>>
>>>> Okay, I found it: recursion-schemes.
>>>>
>>>> On Sat, Oct 22, 2022, 3:05 PM David Feuer <david.feuer at gmail.com>
>>>> wrote:
>>>>
>>>>> That doesn't work out so well when the class has a bunch of methods. I
>>>>> definitely saw the nice way on Hackage ... somewhere.
>>>>>
>>>>> On Sat, Oct 22, 2022, 2:49 PM Brandon Allbery <allbery.b at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> I'm not aware of anything specifically like that, but the `make…`
>>>>>> functions in
>>>>>> https://hackage.haskell.org/package/deriving-compat-0.6.1/docs/Data-Deriving.html
>>>>>> may allow you to do something similar?
>>>>>>
>>>>>> On Sat, Oct 22, 2022 at 2:32 PM David Feuer <david.feuer at gmail.com>
>>>>>> wrote:
>>>>>> >
>>>>>> > I remember seeing a package that offered a function used something
>>>>>> like this:
>>>>>> >
>>>>>> > $(deriveThingy [d| instance Foo a => Thingy a |])
>>>>>> >
>>>>>> > to allow the user to specify the constraint(s) for a generated
>>>>>> instance. I'd love to borrow the code for that, but I can't remember what
>>>>>> package it was in, what class it derived, etc. Can anyone help?
>>>>>> > _______________________________________________
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>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> brandon s allbery kf8nh
>>>>>> allbery.b at gmail.com
>>>>>>
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>>>
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