--- a/src/libasr/ASR.asdl
+++ b/src/libasr/ASR.asdl
@@ -343,6 +343,8 @@ expr
 
 ttype
     = Integer(int kind, dimension* dims)
+    | BitVector(int kind, dimension* dims)
+    | ModularInteger(int kind, dimension* dims)
     | Real(int kind, dimension* dims)
     | Complex(int kind, dimension* dims)
     | Character(int kind, int len, expr? len_expr, dimension* dims)

The bit operations are allowed only on BitVector and we should remove it from the Integer / ModularInteger types. The frontend will insert appropriate implicit casting as needed. Arithmetic operators are defined for both Integer (wrap around is a runtime error in Debug mode) and ModularInteger (that wrap around). Comparison is also defined for both integers.

ModularInteger is not allowed as a loop variable. Initially the ModularInteger would be of modulo 2^8, 2^16, 2^32, 2^64 only, fast on hardware. Later we can add more (not as fast on hardware obviously).

We have to be careful to avoid the pitfalls here: https://github.com/lcompilers/lpython/wiki/ASR-Design#unsigned-integers

The two use cases of BitVector (bit fields) and ModularInteger (modular arithmetic) are good use cases, they should not make things complex. We will see if every other use case of "unsigned integer" can be handled by the combination of these two.