Is there a way to generate instances from the GHCi REPL?
Good evening, Is there a way with TH to generate instances for a class in GHCi? I want to generate instances for a serialization class. I tried to write some TH to do it, but $(foo) in GHCi’s REPL says that it can only be Q Exp, not Q [Dec]. Sadness. I found qAddTopDecls. I tried to add the class declarations via $(foo >>= qAddTopDecls; stringE "OK!") and it says “Only function, value, and foreign import declarations may be added with addTopDecl". Woe. I could create a file and then load in that file, but loading modules loses all GHCi's state, which would defeat the purpose of using a REPL in the first place. Any hope? Ciao!
If your instance-generating function is defined in an external module, you can use it in GHCi like this: |-- Enable TH > :set -XTemplateHaskell -- Import modules (e.g. Aeson) > import Data.Aeson.TH > import Data.Aeson -- Define the type for which I'm going to generate instances > data X = X {x :: Int} -- Derive the instance Prelude Data.Aeson.TH> data Dummy; deriveJSON defaultOptions ''X -- Check that it works > toJSON (X 1) Object (fromList [("x",Number 1.0)]) | (More about the |data Dummy| trick here: https://www.reddit.com/r/haskelltil/comments/3ghacj/you_can_use_template_has....)
One thing to note is that the qAddTopDecls issue isn't a fundamental issue,
just a matter of implementation:
https://ghc.haskell.org/trac/ghc/ticket/10853
On Thu, Feb 4, 2016 at 6:56 AM, Christopher Done
Good evening,
Is there a way with TH to generate instances for a class in GHCi? I want to generate instances for a serialization class.
I tried to write some TH to do it, but $(foo) in GHCi’s REPL says that it can only be Q Exp, not Q [Dec]. Sadness.
I found qAddTopDecls. I tried to add the class declarations via $(foo >>= qAddTopDecls; stringE "OK!") and it says “Only function, value, and foreign import declarations may be added with addTopDecl". Woe.
I could create a file and then load in that file, but loading modules loses all GHCi's state, which would defeat the purpose of using a REPL in the first place.
Any hope?
Ciao!
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participants (3)
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Artyom -
Christopher Done -
Michael Sloan