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

29 Sep 2016

      Frerich Raabe  writes:
...
On 2016-09-28 15:06, Mario Lang wrote:
...
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.
Here's one way to do it (the 'ctorNames' definition is copied out of
your comment):
--- Mario.hs ---
module Mario (makeDotsType) where
import Data.Bits (testBit)
import Language.Haskell.TH
ctorNames :: [String]
ctorNames = "NoDots" : map ctorName [1..63]
  where
    ctorName :: Int -> String
    ctorName = (++) "Dot" . concatMap (show . succ) . flip filter
[0..5] . testBit
makeDotsType :: Q [Dec]
makeDotsType = do
    let ctors = map (\n -> NormalC (mkName n) []) ctorNames
    let instances = map mkName ["Bounded", "Enum", "Eq", "Read", "Show"]
    return [DataD [] (mkName "SixDots") [] ctors instances]
---
--- Main.hs ---
{-# LANGUAGE TemplateHaskell #-}
import Mario
$(makeDotsType)
---
If you compile this with
$ ghc -ddump-splices Main.hs
You can see what type definition that '$(makeDotsType)' expands to.
Oh, thank you!  It could have been so simple...
...
For what it's worth, this may not compile with all versions of the TH
support in GHC; I wrote the above code using GHC 7.10.2.
Works here.
...
In general, I find -ddump-splices invaluable when using TH. I use it
every minute or so to see what code I'm currently generating. What's
noteworthy is that (as mentioned in the 'Using Template Haskell'
section of the GHC user guide) that
You can only run a function at compile time if it is imported from
another module. That is, you can't define
  a function in a module, and call it from within a splice in the same
module.
That's why I used a separate 'Mario' module above.
Yes, I was aware of the need to put the function in a different module.

Thanks again, a working example is really nice to play with.

-- 
CYa,
  ⡍⠁⠗⠊⠕