Dear Emmanuel Chantréau,
You may want to look into Objective CAML http://caml.inria.fr/ which is a French product
as you can see from the Internet address. It is likely better suited to the task
than Haskell and has a reputation for speed. For those who prefer object
oriented programming it has facilities for that which may ease your transition
from C++. The Microsoft F# language is based on Objective CAML.
Haskell has a problem with its type system
and is not rigorous. Haskell is not a suitable language for proof assistants and
so I would advise you to stay clear of Haskell. Standard ML was engineered with
the needs of proof assistants in mind and so you may want to look into Standard
ML, but you should be very happy with Objective CAML. It has an excellent
reputation. The Coq proof assistant which is another French product is based on
Objective CAML.
If you do decide that Haskell is the way, it
will help ease your transition to Haskell. There is nothing that says you can't
keep your fingers in several pies at once.