fp2/week2/mart/StdMaybe.dcl

   1 definition module StdMaybe
   2
   3 //      ********************************************************************************
   4 //      Clean StdLib library module, version 1.0
   5 //      ********************************************************************************
   6
   7 from StdFunc import :: St;
   8 from StdOverloaded import class ==(..);
   9
  10 ::      Maybe x
  11         =       Just x
  12         |       Nothing
  13
  14 isJust          :: !(Maybe .x) -> Bool          // case @1 of (Just _) -> True; _ -> False
  15 isNothing       :: !(Maybe .x) -> Bool          // not o isJust
  16 fromJust        :: !(Maybe .x) -> .x            // \(Just x) -> x
  17
  18 // for possibly unique elements:
  19 u_isJust :: !u:(Maybe .x) -> (!Bool, !u:Maybe .x)
  20 u_isNothing :: !u:(Maybe .x) -> (!Bool, !u:Maybe .x)
  21
  22 accMaybe :: .(St .x .a) !u:(Maybe .x) -> (!Maybe .a,!u:Maybe .x)
  23 // accMaybe f (Just x) = (Just (fst (f x)),Just (snd (f x)))
  24 // accMaybe f Nothing  = (Nothing,Nothing)
  25
  26 mapMaybe        :: .(.x -> .y) !(Maybe .x) -> Maybe .y
  27 // mapMaybe f (Just x) = Just (f x)
  28 // mapMaybe f Nothing  = Nothing
  29
  30 instance ==       (Maybe x) | == x
  31 //      Nothing==Nothing
  32 //      Just a ==Just b <= a==b
  33
  34 maybeToList :: !(Maybe .a) -> [.a];
  35 //      returns list with no or one element
  36
  37 listToMaybe :: ![.a] -> .Maybe .a;
  38 //      returns Just head of list if possible
  39
  40 catMaybes :: ![Maybe .a] -> .[.a];
  41 //      catMaybes ms =  [ m \\ Just m <- ms ]