What I learnd from writing alpm4h

Common patterns in Haskell C FFI design.txt

This is a draft for a future blog post and might be deleted at any time.
Audiances are assumed to be familiar with the c2hs library and Haskell c ffi and have basic knowledge of c.

1. Overall design
I would suggest to do a two-layer design, that is one-level for binding and basic marshalling and one-level for abstraction.

2. Representing C datatypes in Haskell
When designing FFI, you almost always need to represent C datatypes as some Haskell datatypes. You'll need to further add Storable
instance for that Haskell type if you need to marshal between C and Haskell. Associating a C type with Haskell is relatively easy
with c2hs so I'll only cover the marshalling part.

enums are really simple since they are basically CInts, their storable instance can be automated with 
TH(https://github.com/poscat0x04/alpm4h/blob/master/src/Foreign/ALPM/Internal/TH.hs) 
structs without union inside are tricker but can be solved with the derive-storable which utilizes GHC.Generics
unions are a pain in the ass since C doesn't provide any tags to distinguish between each datatype. I failed to find any methods
other than manually writing the Storable instance.

3. Marshalling functions
(1) double pointers: double pointers can be handled with function hooks using "alloca-" and "peek*"
(2)