[Haskell-cafe] Re: static constants -- ideas?

[Don Stewart]

jay:
> Don Stewart dons at galois.com:
> >jay:
> >> I also have constants that are too large to compile. I am resigned to
> >> loading them from data files--other solutions seem even worse.
> ...
> >> Data.Binary eases the irritation somewhat.
> >
> >Did you try bytestring literals (and maybe parsing them in-memory with
> >Data.Binary)?
> 
> That didn't occur to me, since neither of my large constants includes
> strings.... I think you're suggesting that each constant could appear in
> the source as a long bytestring and be deserialized into the data
> structure. If that works, it should improve the startup time, but it's
> still not as nice as simply compiling it straight up.
> 
> I'll try it.

Here's an example, which stores a Data.Map in a gzip-compressed bytestring literal (a C
string literal in the compiled code). The Map is reconstructed on
startup.

    {-# LANGUAGE OverloadedStrings #-}

    import Data.Binary
    import qualified Data.Map as M
    import qualified Data.ByteString.Char8 as S
    import Data.ByteString.Lazy
    import Codec.Compression.GZip

    --
    -- this is a gzip compressed literal bytestring, storing a binary-encoded Data.Map
    --
    mytable =
        "\US\139\b\NUL\NUL\NUL\NUL\NUL\NUL\ETXEN\219\SO\194 \f\197\224\188\196\CAN\227\US\224\171~\NAKc\GS4ce\161`\178\191\215(\176\190\180\167\231\210\n\241\171\203\191\ti\157\217\149\249< \ENQ\214\&9>\202\162\179a\132X\233\ESC=\231\215\164\SYN\157\DC2D\226*\146\174o\t\167\DLE\209\"i_\240\193\129\199<W\250nC\CAN\212\CAN\162J\160\141C\178\133\216;\\@4\144-W\203\209x\205\140\166\RS\163\237]9f\170\143\ACK\163g\223\STX\184\&7\rH\222\FSW\130\&7D\197\NUL\164\&0U\193\186\t\186o\228\180~\NUL\a6\249\137#\SOH\NUL\NUL"

    main = print =<< M.lookup "ghc" m
        where
            -- build the table from the bytestring:
            m :: M.Map String (Maybe String)
            m = decode . decompress . fromChunks . return $ mytable

Running it:

    $ ./A
    Just "dinosaur!"

:)

Important to use a bytestring, since that gets compiled to a C string literal (and not messed
with by the simplifier).

-- Don