[kccqzy]

Nope you didn't understand the situation correctly. First, almost nobody directly parses from a string to JSON AST: people almost always parse into a custom type using either Template Haskell or generics. Second, parsing isn't the issue; doing arithmetic on the number is the issue.

[tome]

Surely the generics approach would go via an aeson Value as an intermediate format, and thus possibly store an unbounded Scientific.

[kccqzy]

Storing it isn't the problem.