Edit TypeNats/AlternativeSingletons authored by diatchki's avatar diatchki
......@@ -5,41 +5,41 @@ We use a family of singleton types to relate type-level naturals to runtime valu
In our design, we chose to provide as primitive an overloaded "smart" constructor and a polymorphic elimination construct:
```wiki
newtype Nat (n :: Nat) = Nat Integer
newtypeT Nat (n :: Nat) = Nat Integer
class NatI n where
nat :: Nat n
tNat :: TNat n
instance NatI 0 where nat = Nat 0
instance NatI 1 where nat = Nat 1
instance NatI 0 where tNat = TNat 0
instance NatI 1 where tNat = TNat 1
...
natToInteger :: Nat n -> Integer
natToInteger (Nat n) = n
tNatInteger :: TNat n -> Integer
tNatInteger (TNat n) = n
```
It is also possible to make the dual choice, where we provide a polymorphic constructor and an overloaded elimination construct:
```wiki
data Nat (n :: Nat) = Nat
data TNat (n :: Nat) = TNat
class NatE n where
natToInteger :: Nat n -> Integer
tNatInteger :: TNat n -> Integer
instance NatE 0 where natToInteger Nat = 0
instance NatE 1 where natToInteger Nat = 1
instance NatE 0 where tNatInteger TNat = 0
instance NatE 1 where tNatInteger TNat = 1
...
```
We made this choice, at least in part, because it made the implementation simpler: the evidence for class `NatI` is just an integer. Note that our choice does not loose any generality because we can define the alternative design in terms of it:
We made this choice, at least in part, because it made the implementation simpler: with our choice the evidence for class `NatI` is just an integer. Note that our choice does not loose any generality because we can define the alternative design in terms of it:
```wiki
data Nat1 (n :: Nat) = Nat
data TNat1 (n :: Nat) = TNat1
nat1ToInteger :: NatI n => Nat1 n -> Integer
nat1ToInteger x = natToInteger (cast nat x)
where cast :: Nat n -> Nat1 n -> Nat n
cast x Nat = x
tNat1Integer :: NatI n => TNat1 n -> Integer
tNat1Integer = tNatInteger . cast
where cast :: NatI n => TNat1 n -> TNat n
cast TNat1 = tNat
```
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