GHC API user: How to stop simplifier from turning recursive let-bindings into mutually recursive functions

Christiaan Baaij christiaan.baaij at
Mon May 8 13:56:59 UTC 2017

Hello GHC Devs,

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

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:

import Control.Applicative

topEntity :: [((),())]
topEntity = (,) <$> outport1 <*> outport2
    (outport1, outResp1) = gpio (decodeReq 1 req)
    (outport2, outResp2) = gpio (decodeReq 2 req)
    ramResp              = ram  (decodeReq 0 req)

    req = core $ (<|>) <$> ramResp <*> ((<|>) <$> outResp1 <*> outResp2)
    {-# INLINE req #-}

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), we can see
that the core-level binder of `topEntity` basically follows the Haskell

However, when we look at the simplifier output, with nearly all
transformations disabled (-O0 -ddump-ds), you will see that parts of
`topEntity` are split into 3 different top-level, mutually recursive,

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.

Kind regards,

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