c - How do I create a storable instance for this structure without c2hs or other tools? -

this structure:

typedef struct atom_ {     float x;     float y;     float z;     int col; } atom; 

corresponds type:

data atom = atom { pos :: v3 float, col :: int } 

how create storable instance atom can send atoms on haskell c function expects atom?

i can't guarantee work shown (i don't have environment set on computer), should first step towards getting right:

import foreign.storable import foreign.ptr  instance storable atom     -- don't make static, let compiler choose depending     -- on platform     sizeof _ = 3 * sizeof (0 :: float) + sizeof (0 :: int)     -- may need fiddle this, , corresponding c     -- code if have ability to, alignment can tricky,     -- in case can pretty safely assume it'll alignment     -- `float`     alignment _ = alignment (0 :: float)     -- these pretty straightforward, , order matters here     -- great deal.  haven't made optimized     -- be, i'm going clarity of code instead     peek ptr =         let floatsize = sizeof (0 :: float)         x   <- peek $ ptr `plusptr` (0 * floatsize)         y   <- peek $ ptr `plusptr` (1 * floatsize)         z   <- peek $ ptr `plusptr` (2 * floatsize)         col <- peek $ ptr `plusptr` (3 * floatsize)         return $ atom (v3 x y z) col     poke ptr (atom (v3 x y z) col) =         let floatsize = sizeof (0 :: float)         poke (ptr `plusptr` (0 * floatsize)) x         poke (ptr `plusptr` (1 * floatsize)) y         poke (ptr `plusptr` (2 * floatsize)) z         poke (ptr `plusptr` (3 * floatsize)) col 

and should work! highly dependent on c compiler , platform, though, you'll want extensive testing make sure it's been laid out correctly.