Default all type variables
Currently not all type variables occurring in a flat curry expression are bound in the type of the surrounding function declaration. Consider the following example:
test = length []
Here, both length
and []
will be typed with a free type variable.
While the resulting FlatCurry Code is certainly correct, all typed back ends need to default these type variables.
If I remember correctly, KiCS2 does not even do this in all required cases, e.g. when the type variable has a kind different than Type
/*
.
We'd like these type variables to be defaulted in the frontend directly.
Currently our idea is to create a new transformation phase on the IL where the AST is traversed and a new data type is created for each unique kind that occurrs in a type variable that needs to be defaulted.
Additionally, each type variable is replaced with the respective type constructor that has been generated.
Note that the generated data types need no constructors. Although it is not possible for a user to write a data type with arbitrary kind but without constructors (all type variables will be phantom and defaulted to *
), we can generate these data types internally.
I am not sure if and when the frontend adds a dummy constructor to external data types or a data type without constructors, but this should be kept in mind.
Also, all environments should be updated with the new data types and they should not be exported to keep the transformation and compilation local and without name clashes.
@fte @mh @fwcd: Anyone got a better Idea?
@fwcd Do you have time to implement this? Should not be too complicated.
Note that in GHC with -XPolyKinds
, Just two data types are sufficient for all possible kinds.
data Zero
data One (a :: k)
In Curry we need:
data Zero
data One (a :: *)
data Two (a :: *) (b :: *)
data OneTwo (a :: * -> *) (b :: *)
...