Version 5 (modified by 8 years ago) (diff) | ,
---|

The only "magic" thing about `GHC.TypeLits`

are the instances of `NatI`

. The rest is implemented like this:

newtype TNat (n :: Nat) = TNat Integer tNatInteger :: TNat n -> Integer tNatInteger (TNat n) = n

So, now we just need instances like these:

instance NatI 0 where nat = TNat 0 instance NatI 1 where nat = TNat 1 instance NatI 2 where nat = TNat 2 ...

Because we cannot generate this infinite family of instances, we have
some code in GHC which can solve `NatI`

predicates on the fly.

The "proof" (aka "dictionary") for `NatI n`

is just the number `n`

. This is OK because:

- GHC uses a
`newtype`

to represent the dictionaries for classes that have just a single method and no super-classes.`NatI`

is just such a class. `TNat`

is already a`newtype`

for`Integer`

.

Therefore, the dictionaries for class `NatI`

are just integers.