3 ms·
What is the small difference?
by melony 5y ago
What is the small difference?
- wyager 5y agoZero-bit busses cannot have an indeterminate value in hardware. However, zero-bit values (Unit) can have indeterminate value in software. If you have the circuit f :: () -> Int f () = 3 Vs g :: () -> Int g _ = 3 Are the exact same thing in hardware but different in software. “f undefined” is undefined in software but “g undefined” is not. Neither is undefined in hardware. This distinction comes up very rarely. Only if you have a parametric circuit that sometimes takes zero-bit inputs (I.e. if the input is unused in some instantiations).
- a1369209993 5y agoIIUC, this is actually a general deficiency in the type system for variable-width tuples. Consider: f1 :: (Bit,()) -> Int f1 (0,()) = 3 f1 (1,()) = 3 -- f1 (0,undefined) is undefined f2 :: (Bit,(Bit,())) -> Int -- and so on. Conversely g2 :: Bit -> Bit -> Int g1 :: Bit -> Int g0 :: Int -- doesn't have this problem but also doesn't work well for busses. -- The 'obvious' approach h3 :: (Bit,Bit,Bit) -> Int h2 :: (Bit,Bit) -> Int h1 :: (Bit) -> Int h0 :: () -> Int -- doesn't work generically; (Bit) isn't a tuple at all, -- and n-element tuples are totally unrelated to m(!=n)-element tuples -- (they're literally declared as: data (a,b) = (a,b) data (a,b,c) = (a,b,c) -- in module GHC.Tuple) You can actually fix this by (ab)using types of kind `TYPE 'UnliftedRep` and unboxed tuples (type `(# Car, Cdr #)`, value `(# b7, b6543210 #)`), it's just generally not worth the effort.