Something with a bit more ML in its blood would be better, and for a library that pretty much means OCaml.
Edit: I do not at all mean to imply that it is impossible to implement TLS securely in Haskell. Only that there are more natural choices if one wants the advantage of a strong algebraic type system.
Here's an example of a type definition that specifies strict evaluation https://github.com/vincenthz/hs-tls/blob/master/core/Network...
See? They don't have to switch languages after all.
The classic example of this is checking string equality with strncmp(): this takes a different amount of time depending on how similar the strings are. If one string is secret and the other is controlled by an attacker, the attacker can use a clock and multiple attempts to discover the secret.
Obviously this particular one isn't relevant to SSL, but there are a number of other possibilities to worry about in most languages, most obviously short-circuiting operations like boolean AND/OR. Lazy evaluation makes every operation short-circuit, so you need to worry about this in every operation.
It can be done, but it's harder than it needs to be.
We could use something that gives both advantages, like the OCaml I already mentioned. Or, we could take a hybrid approach, where something like Haskell generates C code that provably can't have buffer problems. Or, we could statically verify that the library is written in a known-memory-safe subset of C++. Or, we could use a language like Rust, which (once it's eventually complete) seems ideal for this sort of application.