Option to disable turning recursive let-bindings to recursive functions
First some context: I'm using the GHC API to convert Haskell to digital circuit descriptions (clash compiler). When viewed as a structural description of a circuit, recursive let-bindings can be turned into feedback loops. In general, when viewed as a structural description of a circuit, recursive functions describe infinite hierarchy, i.e. they are not realisable as circuit.
So now my problem: the simplifier turns recursive let-bindings to recursive functions; i.e. it is turning something which I can translate to a circuit to something which I cannot translate to a circuit.
Next follows a reduced test case which exemplifies this behaviour:
module Test where
import Control.Applicative
topEntity :: [((),())]
topEntity = (,) <$> outport1 <*> outport2
where
(outport1, outResp1) = gpio (decodeReq 1 req)
(outport2, outResp2) = gpio (decodeReq 2 req)
ramResp = ram (decodeReq 0 req)
req = core $ (<|>) <$> ramResp <*> ((<|>) <$> outResp1 <*> outResp2)
core :: [Maybe ()] -> [()]
core = fmap (maybe () id)
{-# NOINLINE core #-}
ram :: [()] -> [Maybe ()]
ram = fmap pure
{-# NOINLINE ram #-}
decodeReq :: Integer -> [()] -> [()]
decodeReq 0 = fmap (const ())
decodeReq 1 = id
decodeReq _ = fmap id
{-# NOINLINE decodeReq #-}
gpio :: [()] -> ([()],[Maybe ()])
gpio i = (i,pure <$> i)
{-# NOINLINE gpio #-}
Now, when we look at the output of the desugarer (-ddump-ds -dsuppress-all), we can see that the core-level binder of topEntity
basically follows the Haskell code.
topEntity :: [((), ())]
topEntity
= letrec {
ds_d2rI :: ([()], [Maybe ()])
ds_d2rI = gpio (decodeReq 1 req_a2pG);
ds_d2rS :: ([()], [Maybe ()])
ds_d2rS = gpio (decodeReq 2 req_a2pG);
req_a2pG :: [()]
req_a2pG
= $ core
(<*>
$fApplicative[]
(<$>
$fFunctor[] (<|> $fAlternativeMaybe) (ram (decodeReq 0 req_a2pG)))
(<*>
$fApplicative[]
(<$>
$fFunctor[]
(<|> $fAlternativeMaybe)
(case ds_d2rI of { (_, outResp1_X2pR) -> outResp1_X2pR }))
(case ds_d2rS of { (_, outResp2_X2q3) -> outResp2_X2q3 }))); } in
<*>
$fApplicative[]
(<$>
$fFunctor[]
(,)
(case ds_d2rI of { (outport1_a2pB, _) -> outport1_a2pB }))
(case ds_d2rS of { (outport2_a2pN, _) -> outport2_a2pN })
However, when we look at the simplifier output, with nearly all transformations disabled (-O0 -ddump-simpl -dsuppress-all), you will see that parts of topEntity
are split into 3 different top-level, mutually recursive, functions.
Rec {
-- RHS size: {terms: 4, types: 0, coercions: 0, joins: 0/0}
ds_r2so :: ([()], [Maybe ()])
ds_r2so = gpio (decodeReq 1 req_r2sq)
-- RHS size: {terms: 4, types: 0, coercions: 0, joins: 0/0}
ds1_r2sp :: ([()], [Maybe ()])
ds1_r2sp = gpio (decodeReq 2 req_r2sq)
-- RHS size: {terms: 25, types: 50, coercions: 0, joins: 0/0}
req_r2sq :: [()]
req_r2sq
= core
(<*>
$fApplicative[]
(<$>
$fFunctor[] (<|> $fAlternativeMaybe) (ram (decodeReq 0 req_r2sq)))
(<*>
$fApplicative[]
(<$>
$fFunctor[]
(<|> $fAlternativeMaybe)
(case ds_r2so of { (outport1_a2pB, outResp1_X2pR) ->
outResp1_X2pR
}))
(case ds1_r2sp of { (outport2_a2pN, outResp2_X2q3) ->
outResp2_X2q3
})))
end Rec }
-- RHS size: {terms: 13, types: 35, coercions: 0, joins: 0/0}
topEntity :: [((), ())]
topEntity
= <*>
$fApplicative[]
(<$>
$fFunctor[]
(,)
(case ds_r2so of { (outport1_a2pB, outResp1_X2pR) ->
outport1_a2pB
}))
(case ds1_r2sp of { (outport2_a2pN, outResp2_X2q3) ->
outport2_a2pN
})
So my question are:
- Which part of the simplifier is turning these local recursive let-binders into global recursive functions?
- Is there some way to disable this transformation?
- If not, how much effort do you think it would be to put this behaviour behind a dynflag? So that I, as a GHC API user, can disable it for my use-case. I'm willing to implements this dynflag myself.