Clash.Sized.Fixed: fromInteger and fromRational don't saturate correctly

[leonschoorl]

While looking at Ben's on fix for signum Fixed: c2d064f
I noticed that even with that fix:

Clash.Prelude> signum (42 :: SFixed 4 2) == signum (42.0 :: SFixed 4 2)
False

It's because fromInteger(used for the literal 42) and fromRational(used for the literal 42.0) produce very different results when the input is out of range:

Clash.Prelude> 42.0 :: SFixed 4 2
7.75
Clash.Prelude> 42 :: SFixed 4 2
-6.0

But our documentation states that Fixed saturates, so both of these should produces 3 times the same tuple:

Clash.Prelude> [(minBound,maxBound), (-42.0, 42.0), (-42, 42)] :: [(SFixed 4 2,SFixed 4 2)]
[(-8.0,7.75),(-7.75,7.75),(6.0,-6.0)]
Clash.Prelude> [(minBound,maxBound), (-42.0, 42.0), (-42, 42)] :: [(UFixed 4 2,UFixed 4 2)]
[(0.0,15.75),(0.25,15.75),(6.0,10.0)]

[martijnbastiaan]

So I guess this is because Signed and Unsigned don't actually saturate? That is:

Clash.Prelude> 100 :: Signed 2
0

and:

  fromInteger i    = let fSH = fromInteger (natVal (Proxy @frac))
                         res = Fixed (fromInteger i `shiftL` fSH)
                     in res

[martijnbastiaan]

fromInteger does not know anything about its underlying representation, so unless we convert res back to an integer and compare it to the original input I'm not sure how we can fix this.