The instance is defined as
instance KnownBuiltinTypeIn DefaultUni term Integer => ReadKnownIn DefaultUni term Int64 where
readKnown = readKnownAsInteger
where
readKnownAsInteger
:: forall term a
. (KnownBuiltinTypeIn DefaultUni term Integer, Integral a, Bounded a, Typeable a)
=> term -> ReadKnownM a
readKnownAsInteger term =
inline readKnownConstant term >>= oneShot \(i :: Integer) ->
if fromIntegral (minBound :: a) <= i && i <= fromIntegral (maxBound :: a)
then pure $ fromIntegral i
else
throwError . operationalUnliftingError $ <...>While readKnownAsInteger makes sense for most Integral types, it's very inefficient for Int64 specifically, because all you actually need to convert an Integer to Int64 on a 64-bit platform is to unwrap the latter from the IS constructor:
-- If the value is small (i.e., fits into an 'Int'), the 'IS' constructor is
-- used. Otherwise 'IP' and 'IN' constructors are used to store a 'BigNat'
-- representing the positive or the negative value magnitude, respectively.
--
-- Invariant: 'IP' and 'IN' are used iff the value does not fit in 'IS'.
data Integer
= IS !Int# -- ^ iff value in @[minBound::'Int', maxBound::'Int']@ range
| IP !BigNat# -- ^ iff value in @]maxBound::'Int', +inf[@ range
| IN !BigNat# -- ^ iff value in @]-inf, minBound::'Int'[@ range
Currently the ReadKnownIn DefaultUni term Int instance is under the #if WORD_SIZE_IN_BITS == 64 CPP pragma, but that one is guaranteed to be correct without the pragma if you just unwrap from the IS constructor and similarly ReadKnownIn DefaultUni term Word is guaranteed to be correct if you just use readKnownAsInteger there as nearly everywhere else, so I recommend doing it the other way around and deriving the instance for Int64 from the Int one -- and placing the former under the CPP pragma.
The instance is defined as
where
While
readKnownAsIntegermakes sense for mostIntegraltypes, it's very inefficient forInt64specifically, because all you actually need to convert anIntegertoInt64on a 64-bit platform is to unwrap the latter from theISconstructor:Currently the
ReadKnownIn DefaultUni term Intinstance is under the#if WORD_SIZE_IN_BITS == 64CPP pragma, but that one is guaranteed to be correct without the pragma if you just unwrap from theISconstructor and similarlyReadKnownIn DefaultUni term Wordis guaranteed to be correct if you just usereadKnownAsIntegerthere as nearly everywhere else, so I recommend doing it the other way around and deriving the instance forInt64from theIntone -- and placing the former under the CPP pragma.