Write You a Haskell Stephen Diehl

data Kind
= KStar
| KArr Kind Kind
| KPrim
| KVar Name
Since the Core language is explicitly typed, it is trivial to implement an internal type checker for. Running
the typechecker on the generated core is a good way to catch optimization, desugaring bugs, and
determine if the compiler has produced invalid intermediate code.
Type Classes
Typeclasses are also remarkably subtle to implement. We will implement just single parameter typeclasses
and use the usual dictionary passing translation when compiling the frontend to Core. Although the
translation and instance search logic is not terribly complicated, it is however very verbose and involves
a lot of bookkeeping about the global typeclass hierarchy.
For example the following simplified Num typeclass generates quite a bit of elaborated definitions in the
Core language to generate the dictionary and selector functions for the overloaded plus function.
class Num a where
plus :: a -> a -> a
mult :: a -> a -> a
sub :: a -> a -> a
instance Num Int where
plus = plusInt
mult = multInt
sub = subInt
plusInt :: Int -> Int -> Int
plusInt (I# a) (I# b) = I# (plusInt# a b)
is expands into the following set of Core definitions.
plusInt :: Int -> Int -> Int
plusInt = \(ds1 : Int)
(ds2 : Int) ->
case ds1 of {
I# ds8 ->
case ds2 of {
I# ds9 ->
case (plusInt# ds8 ds9) of {
__DEFAULT {ds5} -> (I# ds5)
}
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