Understanding F# seqs - f#

Question

I am trying to use FAKE for automating my project builds. In the process, I wrote the following F# code -
open System
open System.IO
module FileUtils =
type BuildConfiguration = {Name:string; Directory:string}
let DebugBuildConfiguration = {Name = "Debug"; Directory = #"\bin\Debug"}
let ReleaseBuildConfiguration = {Name = "Release"; Directory = #"\bin\Release"}
let StagingBuildConfiguration = {Name = "Staging"; Directory = #"\bin\Staging"}
let codeDir = #"C:\source-control\s4sold\src"
let sourceDirs = [| "co"; "lo"; "mo"; "po"; "pr"; "pro"; "re"; "ro" |]
let GetAllBuildConfigurationDirs buildConfiguration =
let allSourceDirs = sourceDirs
|> Seq.map (fun i -> Path.Combine(codeDir, i))
|> Seq.map (fun d -> Directory.GetDirectories(d))
|> Array.concat
allSourceDirs |> printf "%A"
List.ofArray allSourceDirs
|> List.map (fun i -> Path.Combine(i, buildConfiguration.Directory))
// |> Seq.toArray
Now the problem I am facing is that when I print allSourceDirs, it correctly prints the directories under my /src folder. But when I run GetAllBuildConfigurationDirs, all I get is an array of "\bin\Debug", which means it doesn't take the output of allSourceDirs into consideration.
I am really at a loss to understand what's happening here. I am an F# newbie, but know some Clojure. Also how can I use Seq.map over a seq of seqs (so that I might avoid the Array.concat call)?

Answer 1

The problem is the way you are combining paths, you have to remove the \ at the beginning, just write #"bin\Debug"
Also you can combine both maps into one, since map f |> map g is equivalent to map (f |> g) and instead of map + concat you can use collect.
Here's your original code with these corrections:
open System
open System.IO
module FileUtils =
type BuildConfiguration = {Name:string; Directory:string}
let DebugBuildConfiguration = {Name = "Debug"; Directory = #"bin\Debug"}
let ReleaseBuildConfiguration = {Name = "Release"; Directory = #"bin\Release"}
let StagingBuildConfiguration = {Name = "Staging"; Directory = #"bin\Staging"}
let codeDir = #"C:\source-control\s4sold\src"
let sourceDirs = [| "co"; "lo"; "mo"; "po"; "pr"; "pro"; "re"; "ro" |]
let allSourceDirs = sourceDirs
|> Array.collect (fun i -> Path.Combine(codeDir, i) |> Directory.GetDirectories)
allSourceDirs |> printf "%A"
List.ofArray allSourceDirs
|> List.map (fun i -> Path.Combine(i, buildConfiguration.Directory))
// |> Seq.toArray