Re: Type class instances in scope

19 May 2017

      On 18-05-17 20:41:13, Edward Z. Yang wrote:
...
Hi Tom,
Hi Edward,
...
The problem is that GHC lazily loads non-orphan instances, so they won't
be in the environment until you load the interface which would have
caused the instance to come into scope.
Oh, that's annoying.
I have a feeling there is room for an optimisation here.
... or maybe this was already an optimisation, I don't really know.
...
I'm not sure exactly what you are actually trying to do.
More concretely, I need to generate a line of code for every 'Arbitrary'
instance in scope.

Later I'll also need to use other instances but this is the first part.
...
But if you
really need all instances, you will have to first arrange to load
the interfaces of ALL modules transitively imported by your module.
I don't really mind the time it takes to do this, but that's annoying to
write.

Thank you for your help! I will look into it.
...
Edward
Excerpts from Tom Sydney Kerckhove's message of 2017-05-18 14:39:38 +0200:
...
Dear GHC Devs.
I am trying to use the GHC API as part of the work that I am doing for
my thesis. Currently I am looking for a way to find all the type class
instances that are in scope in a given module.
Here's what I've tried:
```
getInstancesFromTcmodule
    :: GhcMonad m
    => TypecheckedModule -> m ()
getInstancesFromTcmodule tmod = do
    let (tcenv, md) = tm_internals_ tmod
    let insts = tcg_insts tcenv
    getInsts >>= printO
    printO $ modInfoInstances $ tm_checked_module_info tmod
    printO insts
    printO $ md_insts md
    printO $ tcg_inst_env tcenv
printO
    :: (GhcMonad m, Outputable a)
    => a -> m ()
printO a = showGHC a >>= (liftIO . putStrLn)
```
Unfortunately the output that I get is empty:
```
([], [])
[]
[]
[]
[]
```
For the record, I ran this on the following module:
```
{-# LANGUAGE NoImplicitPrelude #-}
module Ints where
import Prelude (Int, (+), (-))
f :: Int -> Int
f x = x + 1
g :: Int -> Int
g x = x - 1
double :: Int -> Int
double x = x + x
zero :: Int
zero = 0
```
Because I'm using '+' and '-', I definitely expect the instances of
'Num' to be available, but I am also expecting to find ALL the other
instances that are available for type checking.
Is there any documentation on this matter?
Failing that, is there anyone who is willing to help me with this
problem?
Thank you for your time.
-- 
Tom Sydney Kerckhove