there are many lovely mutable data structuers in haskell http://hackage.haskell.org/package/vector-0.5/docs/Data-Vect... is one for unboxed C-struct style arrays,
http://hackage.haskell.org/package/hashtables is a super mature stable mutable hashtable library, that is used, among other places, in agda!
..... please fact check your feelings in the future :)