Value restriction in F# -

Value restriction in F# -

i've got f# assignment trying compute transpose of matrix. simple enough, maintain getting value restriction error , cannot figure out why. consulted many of vr error questions out there, i'm still in dark. here's code:

let transpose = function | [xs ; ys] -> allow rec transpose_helper = function | [],[] -> [] | x::xs , y::ys -> [x;y] :: transpose_helper (xs,ys) | _ -> failwith "the matrix not square matrix" transpose_helper(xs,ys) | _ -> failwith "incorrectly formatted input" transpose ([[1;2;3];[2;3;4]]);;

i assume error in part due empty list, nil seems help. pointers appreciated.

edit: next version of code, works. explain why?

let transpose zs = match zs | [xs;ys] -> allow rec transpose_helper (xs, ys) = match (xs,ys) | ([],[]) -> [] | (x::xs , y::ys) -> [x;y] :: transpose_helper (xs,ys) | _ -> failwith "the matrix not square matrix" transpose_helper(xs,ys) | _ -> failwith "incorrectly formatted input" transpose ([[1;2;3];[2;3;4]]);; val transpose : zs:'a list list -> 'a list list val : int list list = [[1; 2]; [2; 3]; [3; 4]]

however, compiler still complains transpose phone call above should have unit type unless bind using lets. please clarify going on here?

my guess wrote let rec transpose zs = match zs ... , later changed utilize function instead, removes need explicit argument. since zs still there function accepts 2 args means you're partially applying it. since zs unused type unknown making partially applied function homecoming generic function value (value restriction). remove zs , well.

f# value-restriction