Question

I was messing around with HashMap and tried to use a Data.Bson.ObjectId as a key. I, of course, discovered that there is not a Hashable instance for that structure. That’s ok, because writing one is trivial.¹

instance Hashable ObjectId where hash (Oid x y) = hash (x,y)

I typed that line into GHCi and was told “parse error on input `instance'”. This actually makes sense as the GHCi prompt operates as if the lines were being typed into a do block in the IO monad and an instance can not be defined in this context.

My question then, is there a way to define a new instance within GHCi?

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¹ Why this instance is not provided by the library is another matter. I would believe the answer was to limit dependencies except that the bson package already depends on everything under the sun.

Answer 1

The good news: Yes, there is a way to define a new instance within GHCi.

The bad news: At the moment, the first step in doing so is “install a development snapshot of GHC”.

This has been an obvious bit of missing functionality in GHCi for quite a while. There was no inherent reason for it to be absent, but I assume it was somewhat difficult to implement and so it got set aside.

However, it seems that as of version 7.4.1, it’s now available:

At the GHCi prompt you can also enter any top-level Haskell declaration, including data, type, newtype, class, instance, deriving, and foreign declarations. For example:

Prelude> data T = A | B | C deriving (Eq, Ord, Show, Enum)
Prelude> [A ..]
[A,B,C]
Prelude> :i T
data T = A | B | C      -- Defined at <interactive>:2:6
instance Enum T -- Defined at <interactive>:2:45
instance Eq T -- Defined at <interactive>:2:30
instance Ord T -- Defined at <interactive>:2:34
instance Show T -- Defined at <interactive>:2:39

Whether you think having that right now worth the hassle of installing a non-release version of GHC is up to you.