Is the list is guaranteed non-empty? Do you want to enforce that at the type level (compile time), or "fail" at runtime when it is?
You should probably be a bit more explicit about what you mean by "concurrently". Do you know in advance that the list length is sufficiently short to make it reasonably to immediately fork an async thread for each?
Also, do you want the outputs to folded in list order, or in any order (e.g. roughly in order of IO action completion)?
The real probleme, is: I have a bunch of hdf5 files which contain a stack of image and other metadata's. for each image an associated metadatas, I can create a cube (3D array), whcih is the binning in the 3D space for each image of the stack read image + metadata -> transformation in 3D space -> binning. then binning -> binning -> binning (this is the monoid), since this is pure computation, I can use unsafe IO to create the merge function. In my case, I want to distribute all this on all my core. each core can do in which ever order a merge of the binning until I have only one binning. [a1, a2, a3, a4] core1: a1 + a2 -> a12 core2: a3 + a4 -> a34 then first core available, a12 + a34 -> a1234 Is it clearer ? Fred