Before you say 'oh, Haskell is still only half the speed of C++', you should keep in mind just how insane the C++ code they had to write looks. The Eigen code they had to write is pretty nasty and getting it correct required some particular understanding.
In comparison, the performance they're getting out of Haskell is coming from comparatively mundane, easy to understand code. That's pretty impressive.
Or another way of putting it: It looks like naive Haskell outperforms naive C++, and naive Haskell is not that much slower than carefully-tuned C++.
template < typename Derived >
typename Derived::Scalar norm2( const MatrixBase <Derived >& v ) {
return v.dot(v);
}