Applying Seq.map using 2 sequences to a method which takes 2 parameters

Question

I'm writing a quick DB perf test, and chose F# so I can get more practice.

I've created a method, measureSelectTimes, which has the signature Guid list * Guid list -> IDbCommand -> TimeSpan * TimeSpan.

Then, I call it:

let runTests () =
    let sqlCeConn : IDbConnection = initSqlCe() :> IDbConnection
    let sqlServerConn : IDbConnection = initSqlServer() :> IDbConnection
    let dbsToTest = [ sqlCeConn; sqlServerConn ]
    let cmds : seq<IDbCommand> = dbsToTest |> Seq.map initSchema
    let ids : seq<Guid list * Guid list> = cmds |> Seq.map loadData
    let input = Seq.zip ids cmds
    let results = input |> Seq.map (fun i -> measureSelectTimes (fst i) (snd i))
    // ...

I've annotated explicitly with types to clarify.

What I can't figure out is how to call measureSelectTimes without the lambda. I'd like to partially apply the ids to it like this: ids |> Seq.map measureSelectTimes but then I don't know what to do with the resulting partially applied functions to then map onto the cmds. What's the syntax for this?

Answer 1

You can use Seq.map2:

Seq.map2 measureSelectTimes ids cmds

Or

(ids, cmds) ||> Seq.map2 measureSelectTimes

Answer 2

Your measureSelectTimes function takes two arguments as separate arguments, but you instead need a function that takes them as a tuple. One option is to just change the function to take a tuple (if it is logical for the arguments to be tupled).

Alternative, you can write a cobinator that turns a function taking two arguments into a function taking tuple. This is usually called uncurry and it exists in some functional language:

let uncurry f (a, b) = f a b

Then you can write:

input |> Seq.map (uncurry measureSelectTimes)

This looks okay for a simple use like this, but I think that using combinators too much in F# is not a good idea as it makes code difficult to read for less experienced functional programmers. I would probably write something like this (because I find that more readable):

[ for (time1, time2) in input -> measureSelectTimes time1 time2 ]

Answer 3

One approach is to change the signature of measureSelectTimes to

(Guid list * Guid list) * IDbCommand -> TimeSpan * TimeSpan

Then you can change the map call to

let results = input |> Seq.map measureSelectTimes
// or
let results = Seq.map measureSelectTimes input