Defining storable instance for union data types

Question

How do you define the storable vector instance for a data type like below (composed from GHC primitive types):

data Atoms =  I GHC.Int.Int32|S GHC.Int.Int16 -- define a union data type

I checked this storable tutorial but it works only for vectors of same types, not union like above.

dflemstr dflemstr · Accepted Answer · 2011-12-08T22:03:52

You have to encode which constructor you used to instantiate the type somehow.

You can for example add a byte that specifies the index of the constructor that was used. This means that the values above could be stored like this:

Haskell    Binary
I 3     -> 00 00 00 00 03
S 4     -> 01 00 04 XX XX
              ^ Data
           ^ Constructor index
XX = unused byte

Then, when you want to deserialize a value from a byte string, you peek the first byte, see which index it is, and choose the constructor to use (and what to peek off next) based on that.

Defining storable instance for union data types

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