Question

Messing around with ‘extension functions’ for the List module. (I spent quite a while developing ‘mapfold’ – which threads an accumulator like fold, but uses it as a parameter to create new values like map – then discovered that that is what List.scan_left does)

For generation of test data, I needed to do a cross product of two lists, This is what I came up with:

///Perform cross product of two lists, return tuple let crossproduct l1 l2 =     let product lst v2 = List.map (fun v1 -> (v1, v2)) lst     List.map_concat (product l1) l2 

Is this any good, or is there already some better way to do this?

Same question for this one:

///Perform cross product of three lists, return tuple let crossproduct3 l1 l2 l3 =     let tuplelist = crossproduct l1 l2 //not sure this is the best way...     let product3 lst2 v3 = List.map (fun (v1, v2) -> (v1, v2, v3)) lst2     List.map_concat (product3 tuplelist) l3 

Answer 1

another option is to use F# ‘sequence expressions’ and write something like this:

let crossproduct l1 l2 =   seq { for el1 in l1 do           for el2 in l2 do             yield el1, el2 };; 

(actually, it is almost the same thing as what you wrote, because ‘for .. in .. do’ in sequence expression can be viewed as map_concat). This works with (lazy) sequences, but if you want to work with lists, you’d just wrap the code inside [ … ] rather than inside seq { … }.