Re: [Haskell-cafe] Help with TH trick

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 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 wrote:

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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 wrote:

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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 wrote:

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Okay, I found it: recursion-schemes.

On Sat, Oct 22, 2022, 3:05 PM David Feuer wrote:

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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 wrote:

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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... may allow you to do something similar?

On Sat, Oct 22, 2022 at 2:32 PM David Feuer 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? > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post.

-- brandon s allbery kf8nh allbery.b@gmail.com

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