Is Prep supposed to keep thing in (reverse) dependency order?

3 Nov 2022

      Hi,

I was under the impression that the Prep'd Core is still in reverse 
dependency order, i.e. if definition A refers to intra-module definition 
B, then A's binding will always be preceding B's binding. I've been 
exploiting this (hopefully not made-up and accidental!) property in my 
work so far.

However, for the following input:

```

$s$fFunctorFun_$sfmap
   :: forall a b.
      (a -> b)
      -> Fun '[AnnexType, CSASource, Currency, String] a
      -> Fun '[AnnexType, CSASource, Currency, String] b
$s$fFunctorFun_$sfmap
   = (\ (@a)
        (@b)
        (f :: a -> b)
        (g :: Fun '[AnnexType, CSASource, Currency, String] a)
        (x1 :: AnnexType) ->
        let {
          g1 :: TFun '[CSASource, Currency, String] a
          g1
            = (g `cast` Co:32) x1 } in
        (\ (x2 :: CSASource) ->
           $s$fApplicativeFun_$s$fFunctorFun2
             @Currency
             @a
             @b
             f
             (((g1 `cast` Co:55) x2)
           `cast` Co:22)
        `cast` Co:148)
     `cast` Co:125)
```

I am getting the following output from Prep:

```
sat_s2iCp
   :: forall a b.
      (a -> b)
      -> Fun '[AnnexType, CSASource, Currency, String] a
      -> Fun '[AnnexType, CSASource, Currency, String] b
sat_s2iCp
   = (\ (@a)
        (@b)
        (f :: a -> b)
        (g :: Fun '[AnnexType, CSASource, Currency, String] a)
        (x1 :: AnnexType) ->
        let {
          g1 :: TFun '[CSASource, Currency, String] a
          g1
            = (g `cast` Co:32) x1 } in
        let {
          sat_s2iCo :: CSASource -> Currency -> Fun '[String] b
          sat_s2iCo
            = \ (x2 :: CSASource) ->
                $s$fApplicativeFun_$s$fFunctorFun2
                  @Currency
                  @a
                  @b
                  f
                  (((g1 `cast` Co:55) x2) `cast` Co:22) } in
        sat_s2iCo
        `cast` Co:148)
     `cast` Co:125

$s$fFunctorFun_$sfmap
   :: forall a b.
      (a -> b)
      -> Fun '[AnnexType, CSASource, Currency, String] a
      -> Fun '[AnnexType, CSASource, Currency, String] b
$s$fFunctorFun_$sfmap
   = \ (@a)
       (@b)
       (eta_B0 :: a -> b)
       (eta_B1 :: Fun '[AnnexType, CSASource, Currency, String] a) ->
       sat_s2iCp @a @b eta_B0 eta_B1
```

Note that the body of `$s$fFunctorFun_$sfmap` has been lifted out to 
`sat_s2iCp`, but the binding of `sat_s2iCp` precedes the binding of 
`$s$fFunctorFun_$sfmap`.

Is this expected, for some reason, or is this a GHC bug? If the latter, 
I'm happy to file a ticket but I can't promise an actual end-to-end 
compilable reproducer...

Is Prep supposed to keep thing in (reverse) dependency order?

ÉRDI Gergő