Nim example of Haskell Maybe type

gistfile1.nim

###############################                                                 
## Library code                                                                 
                                                                                
type                                                                            
  TMaybeKind = enum                                                             
    mkJust,                                                                     
    mkNothing                                                                   
  TMaybe[T] = object                                                            
    case kind: TMaybeKind                                                       
    of mkJust: val: T                                                           
    of mkNothing: noVal: int                                                    

proc eval[T](a: TMaybe[T], justProc: proc (x: T), nothingProc: proc ()) =       
  case a.kind                                                                   
  of mkJust: justProc(a.val)                                                    
  of mkNothing: nothingProc()                                                   

proc just[T](v: T): TMaybe[T] = TMaybe[T](kind: mkJust, val: v)                 
proc nothing(): TMaybe[int] = TMaybe[int](kind: mkNothing, noVal: 0)            

###############################                                                 
## Application code                                                             

proc prVal[T](val: T) = echo "val is " & $val                                   
proc prNo() = echo "no val"                                                     

let                                                                             
  a = just(7)                                                                   
  b = nothing()                                                                 
  c = just("hello world")                                                       

eval(a, prVal[int], prNo)                                                       
eval(b, prVal[int], prNo)                                                       
eval(c, prVal[string], prNo)