A CIL Tutorial - Department of Computer Science - ETH Zürich

CHAPTER 7. TYPE QUALIFIERS 54
let colors of type (t : typ) : color list =
color strings
|> L.filter (isTypeColor t)
|> L.map color of string
7.1.2 Type-checking
Now that we can extract the qualiers we care about from a type, we can check that two types are
compatible. First we dene a type codifying the results of comparing two types, typecheck result.
Then, we'll write a function that performs the comparison, checkColorTypes. TypesMismatch and
ColorsMismatch are parameterized by the types that didn't match. This is to enable writing better
error and warning messages.
type typecheck result =
| TypesOkay
| TypesMismatch of typ × typ
| ColorsMismatch of typ × typ
The function colorTypesCompat rst ensures that two types have the same colors before descending
recursively into the structure of a type. That is, we check that pointer types have the same colors
before checking that the pointed-to types also have the same colors. We'll leave as exercises the
correct handling of function types, and also make some suggestions about how to modify this code
to make it more general-purpose.
let rec colorTypesCompat (t1 : typ) (t2 : typ) : typecheck result =
let cl1 = colors of type t1 in
let cl2 = colors of type t2 in
if cl1 ≠ cl2 then ColorsMismatch(t1, t2) else begin
match t1, t2 with
| TVoid , TVoid → TypesOkay
| TPtr(t1, ), TPtr(t2, )
| TArray(t1, , ), TArray(t2, , ) → colorTypesCompat t1 t2
| TFun , TFun → TypesOkay (∗ See the exercise below ∗)
(∗ ... ∗)
| , → TypesMismatch(t1, t2)
end
The function warning for tcres generates warning messages for the various kinds of typecheck results.
In certain situations, some mismatches may be okay. To handle that, one would extend this function
with a second parameter that indicates which mismatches are okay in a particular situation.