[Haskell-beginners] A seemingly simple use-case for Template Haskell

Wed Sep 28 14:18:28 UTC 2016

Hi Mario,

I had a similar itch to scratch when I first started noodling with Haskell, coming from Lisp, and wrote the following for generating enums from a convenient notation:

https://github.com/yesodweb/yesod/wiki/Create-String-Based-Enums-With-Template-Haskell <https://github.com/yesodweb/yesod/wiki/Create-String-Based-Enums-With-Template-Haskell>

It’s approaching four years old, so likely no longer works as is, but I hope it is helpful.

Even more than with Lisp macros, TH should be seen as a last resort. However, sometimes you gotta scratch. I was frustrated at the time that I couldn’t find any decent examples, and I specifically wanted to learn some TH - regardless of whether there was a better way available.

In that wiki page you’ll find code for a custom quasi quoter, and code for generating declarations. I mostly just relied on the template haskell docs:

https://hackage.haskell.org/package/template-haskell <https://hackage.haskell.org/package/template-haskell>

which I found to have everything I needed, though of course there is a lot to digest in there.

After gaining some familiarity, the types and functions for creating those types feel a natural match to regular Haskell source, and Q is used because otherwise your declarations would be just side effect free values, unable to affect your program.

For your needs, find “data Dec” in the template haskell docs, and you’ll see that `DataD` matches what you need at term level. Then take a look at `dataD`, a function that will construct your declaration, using Q.

Cheers,
Si

> On 28 Sep 2016, at 15:06, Mario Lang <mlang at delysid.org> wrote:
> 
> Hi.
> 
> As a long-term Lisp fan, and someone who always admired compile-time
> code-generation, I wanted to at least do something simple with Template
> Haskell once.
> 
> In a small project of mine, I have this basically auto-generated data
> type:
> 
> -- Braille music code only uses the old 6-dot system.  We enumerate all
> -- possible dot patterns to use the type system to avoid accidentally
> -- specifying invalid dot patterns in the source code.
> --
> -- gen :: String
> -- gen =
> --     "data Braille = " ++ intercalate " | " ctors ++ " deriving (Enum, Eq)" where
> --   ctors = "NoDots" : map ctorName [1..63] where
> --     ctorName :: Int -> String
> --     ctorName = (++) "Dot" . concatMap (show . succ) . flip filter [0..5] . testBit
> 
> data SixDots = NoDots | Dot1 | Dot2 | Dot12 | Dot3 | Dot13 | Dot23 | Dot123
>             | Dot4 | Dot14 | Dot24 | Dot124 | Dot34 | Dot134 | Dot234
>             | Dot1234 | Dot5 | Dot15 | Dot25 | Dot125 | Dot35 | Dot135
>             | Dot235 | Dot1235 | Dot45 | Dot145 | Dot245 | Dot1245 | Dot345
>             | Dot1345 | Dot2345 | Dot12345 | Dot6 | Dot16 | Dot26 | Dot126
>             | Dot36 | Dot136 | Dot236 | Dot1236 | Dot46 | Dot146 | Dot246
>             | Dot1246 | Dot346 | Dot1346 | Dot2346 | Dot12346 | Dot56 | Dot156
>             | Dot256 | Dot1256 | Dot356 | Dot1356 | Dot2356 | Dot12356
>             | Dot456 | Dot1456 | Dot2456 | Dot12456 | Dot3456 | Dot13456
>             | Dot23456 | Dot123456
>             deriving (Bounded, Enum, Eq, Read, Show)
> 
> So, while actually quite simple, this looks like an opportunity to use
> Template Haskell for me.  In other words, I want to figure out what is
> necessary to generate this data type with TH, instead of the gen
> function that basically generates a piece of plain Haskell code.
> 
> I have been reading "A practical Template Haskell Tutorial"[1] but I find
> it a little bit too terse to actually solve this very little riddle on
> my own.
> 
> For one, I find it confusing that some TH functions return "Q Dec" while
> others just return Dec.  I am aware that this is some sort of Monad for
> the TH system, but I have never seen it explained anywhere.
> 
> Also, all the examples I can find seem to be mostly focused in
> generating Q Exp or similar, but I didn't really find an example
> for Q Dec.
> 
> I realize this should be simple to figure out on my own, but it
> apparently is not.  I have tried to wrap my head around this on my own
> at least three times now, but always stopped after an hour or two due to
> frustration.  Is there some comprehensive TH documentation I haven't
> seen yet?  Could you please give me enough of a head-start that I
> actually manage to write something which can generate this simple data
> type above?
> 
> [1] https://wiki.haskell.org/A_practical_Template_Haskell_Tutorial
> -- 
> CYa,
>  ⡍⠁⠗⠊⠕
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