Simulator throws an exception instead of returning X values on undefined RAM signal

[gergoerdi]

It seems in certain circumstances, undefined values in RAM spoil the whole thing, not even into a fully-undefined signal, but one that throws an exception.

Here's a repro case on 0.2.4. Consider the following module:

import Language.KansasLava hiding (D)
import Language.KansasLava.Signal
import Data.Sized.Unsigned
import qualified Language.KansasLava.Stream as Stream

type A = U4
type D = U8

boolS :: Signal CLK Bool
boolS = iterateS bitNot True

toRV :: (Rep a, Clock clk) => Signal clk a -> RepValue
toRV = toRep . (!!0) . Stream.toList . shallowS

rv1 :: RepValue
rv1 = toRV (pack (undefinedS, boolS) :: Signal CLK (A -> D, Bool))

rv1' :: RepValue
rv1' = toRV (pack (undefinedS, undefinedS) :: Signal CLK (A -> D, Bool))

If we look at rv1 and rv1', we can see that rv1' takes on undefined (X) values as expected:

λ» rv1'
0bXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

However, rv1 crashes:

λ» rv1
0b1*** Exception: ("X",16[8U],"15")

[gergoerdi]

Note that this bug doesn't affect all "compound" signal types:

rv2 :: RepValue
rv2 = toRV (pack (undefinedS, boolS) :: Signal CLK (Matrix A D, Bool))

rv2' :: RepValue
rv2' = toRV (pack (undefinedS, undefinedS) :: Signal CLK (Matrix A D, Bool))

this results in both rv2 and rv2' being correct:

λ» rv2
0bXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
λ» rv2'
0bXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

[gergoerdi]

(apologies for the accidental close-and-reopen)

[gergoerdi]

The above suggests a workaround: if we have

funS :: Signal CLK (A -> D)
funS = undefinedS

s :: Signal CLK (A -> D, Bool)
s = pack (funS, boolS)

s' :: Signal CLK (Matrix A D, Bool)
s' = pack (bitwise funS, boolS)

then toRV s' works:

λ» toRV s
0b1*** Exception: ("X",16[8U],"15")
λ» toRV s'
0bXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

[gergoerdi]

The exception is coming from the Rep instance of ix -> a:

instance (Size ix, Rep a, Rep ix) => Rep (ix -> a) where
    data X (ix -> a) = XFunction (ix -> X a)

    unX (XFunction f) = return (\ a -> 
        let fromJust' (Just x) = x
            fromJust' _ = error $ show ("X",repType (Witness :: Witness (ix -> a)), showRep (optX (Just a) :: X ix))
        in (fromJust' . unX . f) a)

There's no easy way to fix this since unX has to return a Maybe (ix -> a) instead of ix -> Maybe a which would work better here.