GHCi command, tracing steps of instance resolution for Constraint or expression
Another GHCi command (#15610), `:elab <constraint>` traces instance resolution for `<constraint>`. This is already something people do by hand (ticket:10318\#[ticket:15613\#comment:159914](https://gitlab.haskell.org//ghc/ghc/issues/15613#note_159914)) and would be a great tool for explorers of Haskell
```
>> :elab Monoid (a -> b -> ([c], Sum Int))
Monoid (a -> b -> ([c], Sum Int))
==> Monoid (b -> ([c], Sum Int))
==> Monoid ([c], Sum Int)
==> Monoid [c]
==> Monoid (Sum Int)
==> Num Int
```
If resolving the type class fails, it can pinpoint what caused it to fail
```
>> data A
>> :elab Show (A, Int -> Int)
Show (A, Int -> Int)
<~bRZsz NO instance~>
==> Show A
<NO instance>
==> Show (Int -> Int)
<NO instance>
```
A verbose version can explain each step
```
>> :elab +v Monoid (a -> b -> ([c], Sum Int)
Monoid (a -> b -> ([c], Sum Int)) -- Monoid b => Monoid (a -> b) (‘GHC.Base’)
==> Monoid (b -> ([c], Sum Int)) -- Monoid b => Monoid (a -> b) (‘GHC.Base’)
==> Monoid ([c], Sum Int) -- Monoid b => Monoid (a -> b) (‘GHC.Base’)
==> Monoid [c] -- Monoid [a] (‘GHC.Base’)
==> Monoid (Sum Int) -- Num a => Monoid (Sum a) (‘Data.Monoid’)
==> Num Int -- Num Int (‘GHC.Num’)
```
```
>> :elab +v Num (Int, Float, Rational)
Num (Int, Float, Rational) -- (Num a, Num b, Num c) => Num (a, b, c) (‘Data.NumInstances.Tuple’)
==> Num Int -- Num Int (‘GHC.Num’)
==> Num Float -- Num Float (‘GHC.Float’)
==> Num Rational -- type Rational = Ratio Integer (‘GHC.Real’)
= Num (Ration Integer) -- Integral a => Num (Ratio a) (‘GHC.Real’)
==> Integral Integer -- Integral Integer (‘GHC.Real’)
```
----
Not the same idea but similar. Listing instance resolution that takes place in an expression
```
>> :elab (+) @Int
from: (+) @Int
Num Int
```
```
>> :elab2 comparing (length @[]) <> compare
from: length @[]
Foldable []
from: comparing (length @[])
Ord Int
from: comparing (length @[]) <> compare
Monoid ([a] -> [a] -> Ordering)
==> Monoid ([a] -> Ordering)
==> Monoid Ordering
```
```
>> :elab2 ask 'a'
from: ask 'a'
MonadReader Char ((->) m)
==> MonadReader Char ((->) Char)
```
not sure about that last one, or how to visualize them but I think it gives the right idea.
Make sure to test it on https://jpaykin.github.io/papers/paykin_dissertation_2018.pdf
```hs
lam
:: (HasLolli exp, KnownNat n,
Div_ (Remove n ctx') (Remove n ctx') ~ '[],
Fresh (Remove n ctx') ~ n,
MergeF (Remove n ctx') '[] ~ Remove n ctx', MergeF ctx' '[] ~ ctx',
Lookup ctx' n ~ 'Just a, AddF n a (Remove n ctx') ~ ctx',
Div_ ctx' ctx' ~ '[]) =>
(Var exp n a -> exp ctx' b) -> exp (Remove n ctx') (a -· b)
```
and [this](https://www.reddit.com/r/haskell/comments/ahu6jp/fun_fact_the_continuation_monad_cont_r_a_has_an/eejuhlq/)
issue