Overcoming limitation with polymorphic type in haskell -

i have type so,

data agentspec = agentspec                       { agent ::                      , events :: [bytestring]                       } deriving (show) 

and other types so,

data java = java data php = php 

i trying create function of type,

allevents :: [agentspec a] -> [bytestring] allevents aspec = concatmap events aspec 

but unable create values of type [agentspec a]

if do,

allevents [agentspec java ["event1, "event2"], agentspec php ["event3, "event4"]] 

it not typecheck , understandably so. doesn't type check because, agentspec java not agentspec a

so understand why not work. not know how overcome limitation without writing lot of duplicate code.

one alternatve manually construct list of type,

allevents :: [bytestring] allevents = ["event1", "event2", "event3", "event4"]. 

but feel i'm rewriting things i've modelled in types. there way make use of existing types want? concatenated list of bytestrings of agentspecs

this code:

allevents [agentspec java ["event1, "event2"], agentspec php ["event3, "event4"]] 

doesn't compile because agentspec java ["event1, "event2"] , agentspec php ["event3", "event4"]are of different types, , lists in haskell can contain 1 type. agentspec [bytestring] can created type a, once created can't mixed values of different type.

i don't know you're modeling, typically i'd recommend this:

data language = java | php  data agentspec = agentspec                       { agent :: language                      , events :: [bytestring]                       } deriving (show)  allevents = [agentspec java ["event1", "event2"], agentspec php ["event3", "event4"]] 

based on comment writing library, though, doesn't sound work. can elaborate on you're trying achieve?