loops (hackage): loops is a library for fast, imperative-style loops with a clean syntax.  Features  Fast, imperative-style loops with a clean syntax. Bind (>>=) nests loops, so in do-notation, each subsequent line is nested inside loops that appear above it.  Iteration over common data structures, like lists and vectors.  Robust performance because there is no reliance on fusion.  NEW! Loop-unrolling to arbitrary depth. Unrollable loop combinators are provided in Control.Monad.Loop.Unroll. (The simple, "rolled" interface is still provided in Control.Monad.Loop.) The unrolling depth set at the call site at compile time. My benchmarks show that folding over unrolled loops is up to 25% faster than folding over unboxed vectors!  For best performance, please compile your code with -O2. You should also use GHC's LLVM backend if possible; it generally produces faster executables.
Keywords: bsd3|control|library