haskell - Adding a new constructor to a datatype after declaration -

i have requirement add new constructors datatype after it's module has been loaded. pseudo code may seem this:

import mymodule  data mytype = newconstructor

in above example mytype has been defined in mymodule. there way that?

you can't. datatype closed. , thing otherwise, how can predict how previsouly defined function behave?

several workarounds exist, here off top of head:

use function constructor. e.g. myextendedtype = mytype 42;
use own type , mapping function. e.g. data myextendedtype = myextendedtype; tomytype myextendedtype = mytype 42;

wrap existing datatype:

import mymodule old  data myextendedtype = fromold (mytype int) | myextendedtype  foo (fromold m) = old.foo m foo myextendedtype = undefined

Error