Type mismatch Seq.unfold

Question

let fibgen (x,y) = if(x < 4000000) then Some(x+y, (y, x+y)) else None

let fibseq = Seq.unfold fibgen (1,1)

Type Mismatch error on Second line. what am I doing wrong? I am using F# 2.0

Answer

First Done Reset Session on My interactive Window still same error
and then Restarted My Project now Working fine.

yes i had below piece of code execued in same session

// The option type is a discriminated union.
type Option<'a> =
    | Some of 'a
    | None




Interactive window Output as below

val fibgen : int * int -> Option<int * (int * int)>

> 


Eluer2.fs(27,25): error FS0001: Type mismatch. Expecting a
    int * int -> ('a * (int * int)) option    
but given a
    int * int -> Option<int * (int * int)>    
The type '('a * (int * int)) option' does not match the type 'Option<int * (int * int)>'
> 

Answer 1

As pointed by Carsten in a comment, the code works fine if you enter it in clean F# Interactive. Since the compiler complains that Option<'T> does not match 'T option, I would guess that you accidentally wrote some code that re-defined Some and None before using the unfold function. Perhaps you wrote something like this (and evaluated that in F# Interactive):

type Option<'T> = 
  | None
  | Some of 'T

This hides the standard definitions of Some and None that are constructors of the 'T option type defined in the standard F# library (and that are expected by the unfold function).

You can reset the F# Interactive session by right-clicking in the window and choosing "Reset Session". This removes previous declarations and the code should work fine.