Structure Option

(* Option -- SML Basis Library *)

exception Option

datatype option = datatype option

val getOpt         : 'a option * 'a -> 'a 
val isSome         : 'a option -> bool 
val valOf          : 'a option -> 'a 
val filter         : ('a -> bool) -> 'a -> 'a option 
val map            : ('a -> 'b) -> 'a option -> 'b option
val app            : ('a -> unit) -> 'a option -> unit
val join           : 'a option option -> 'a option
val compose        : ('a -> 'b) * ('c -> 'a option) -> ('c -> 'b option)
val mapPartial     : ('a -> 'b option) -> ('a option -> 'b option)
val composePartial : ('a -> 'b option) * ('c -> 'a option) -> ('c -> 'b option)

(* 
   [getOpt (xopt, d)] returns x if xopt is SOME x; returns d otherwise.

   [isSome vopt] returns true if xopt is SOME x; returns false otherwise.

   [valOf vopt] returns x if xopt is SOME x; raises Option otherwise.

   [filter p x] returns SOME x if p x is true; returns NONE otherwise.

   [map f xopt] returns SOME (f x) if xopt is SOME x; returns NONE otherwise.

   [app f xopt] applies f to x if xopt is SOME x; does nothing otherwise.

   [join xopt] returns x if xopt is SOME x; returns NONE otherwise.

   [compose (f, g) x] returns SOME (f y) if g x is SOME y; returns NONE 
   otherwise.  It holds that compose (f, g) = map f o g.

   [mapPartial f xopt] returns f x if xopt is SOME x; returns NONE otherwise.  
   It holds that mapPartial f = join o map f.

   [composePartial (f, g) x] returns f y if g x is SOME y; returns NONE 
   otherwise.  It holds that composePartial (f, g) = mapPartial f o g.

   The operators (map, join, SOME) form a monad.
*)