Haskell Data.Vector.Storable.unsafeFromForeignPtr with C struct pointer/array field -

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i'm using haskell ffi c library defines number of struct types containing members pointers doubles, intended treated arrays of doubles:

typedef struct foo {   int length;   double* values; } foot;

in haskell bindings library have equivalent data type in i'm trying use data.vector.storable.vector double array:

data foo = foo {   length :: int,   values :: data.vector.storable.vector double } deriving (show, eq)

in order marshall data between c library , haskell code, of course, have write storable instances these types. i'm trying work out way of using data.vector.storable.unsafefromforeignptr create haskell vectors double* arrays c library has allocated , populated on heap. i'm hoping doing can avoid copying contents of double* arrays , have vector kind of wrapper on array. (side question be: given double* arrays can 10,000s of doubles, worth pursuing non-copying?)

this have far. i'm using hsc2hs macros generate storable peek implementation:

instance storable foo   alignment _ = alignment (undefined :: cdouble)   sizeof _ = #{size foot}    peek ptr =     len <- (#peek foot, length) ptr     valuesfield <- ((#peek foot, values) ptr) :: io (foreignptr double)     let values' = dv.unsafefromforeignptr0 valuesfield len      return foo { length = len, values = values' }    poke ptr (foo len values') =     (#poke foot, length) ptr len     dv.unsafewith values' (\ptrvalues -> (#poke foot, values) ptr ptrvalues)

so in peek i'm trying #peek values member foreignptr double use unsafefromforeignptr. however, #peek generates code this:

valuesfield <- (((\ hsc_ptr -> peekbyteoff hsc_ptr 16)) ptr) :: io (foreignptr double)

and gets stuck because there's no storable instance foreignptr double. think if tried implement instance foreignptr double commuting problem of how access address value of struct member peek implementation instance.

so in summary, how can access address value (i.e. pointer) struct member in such way can use argument unsafefromforeignptr?

i don't know using hsc2hs have pointer data in peek use that, proper offset of course. disclaimer: compiles, untested. 

import data.vector.storable (vector, unsafefromforeignptr) import foreign.storable (storable (..)) import foreign.c.types  import foreign.foreignptr (newforeignptr_) import foreign.ptr   instance storable foo   peek ptr =      len <- peek (castptr ptr)     valsptr <- newforeignptr_  (castptr ptr `plusptr` (sizeof (undefined :: cint)))     return $ foo len $ unsafefromforeignptr valsptr 0 len

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