[Haskell-cafe] Best way to build strings?

[Jonathan Cast]

Albert Lai <trebla at vex.net> wrote:
<snip>
> I wish  to toss out  a new thought.   To that end  let me blow  up the
> example to underline a scalability issue:
> 
> A. q ++ " " ++ a ++ " " ++ z ++ " [" ++ m ++ " -> " ++ k ++ " |" ++ p ++ "| "
>    ++ g ++ " -> " ++ c ++ "] " ++ h ++ " " ++ b ++ " " ++ f ++ " " ++ i
> B. printf "%s %s %s [%s -> %s |%s| %s -> %s] %s %s %s %s" q a z m k p g c h
>    b f i
> 
> B looks  clearer because without parsing  you can see  that the output
> will contain a |blah| between two blah->blah's inside square brackets,
> etc.
> 
> A looks  clearer because without  counting you can  see that p  is the
> thing that  will go  into |blah|, the  first blah->blah will  be m->k,
> etc.
> 
> The best of both worlds may  be something like the notation in the HOL
> theorem prover:
> 
> ``^q ^a ^z [^m -> ^k |^p| ^g -> ^c] ^h ^b ^f ^i``
> 
> Do you agree that this is much better?
> 
> Could someone implement something like this in GHC please? :)

Don't have time to work it out in detail, but something like this sounds
promising:

> lexToken :: String -> alpha -> (String -> String -> alpha) -> alpha
> lexToken (c:s) f x
>   | isAlpha c || c == '_'
>   = flip fix ([c], s) $ \ loop (s1, s2) -> case s2 of
>       (c:s2') | isAlphaNum c || c == '_' || c == '\''
>             -> loop (c:s1, s2')
>       _ -> f (reverse s1) s2
> lexToken _ f x = x

> interpolate :: String -> Q Exp
> interpolate [] = listE []
> interpolate ('^':s)
>   = lexToken s (fail "Expected valid Haskell identifier") $ \ s1 s2 ->
>       infixE (Just (varE 'show `appE` varE (mkName s1)))
>              (varE '(++))
>              (Just (interpolate s2))
> interpolate s = let
>      (s1, s2) = break (=='^') s
>   in infixE (Just (litE $ StringL s1))
>             (varE '(++))
>             (Just (interpolate s2))

Call as $(interpolate "^q ^a ^z [^m -> ^k |^p| ^g -> ^c] ^h ^b ^f ^i").

Note: this is untested  code!  Be sure and test it before  you use it (I
haven't even compiled it).

Confident someone can do better (e.g., formatting parameters, quoting),

Jon Cast