src/hope_option.erl

   1 -module(hope_option).
   2
   3 -behavior(hope_monad).
   4
   5 -export_type(
   6     [ t/1
   7     ]).
   8
   9 -export(
  10     [ put/2
  11     , get/2
  12     , return/1
  13     , map/2
  14     , iter/2
  15     , pipe/2
  16     , of_result/1
  17     , of_undefined/1
  18     ]).
  19
  20
  21 -type t(A) ::
  22       none
  23     | {some, A}
  24     .
  25
  26
  27 -spec put(A, fun((A) -> boolean())) ->
  28     t(A).
  29 put(X, F) ->
  30     case F(X)
  31     of  true  -> {some, X}
  32     ;   false -> none
  33     end.
  34
  35 -spec get(t(A), Default :: A) ->
  36     A.
  37 get({some, X}, _) -> X;
  38 get(none     , Y) -> Y.
  39
  40 -spec return(A) ->
  41     {some, A}.
  42 return(X) ->
  43     {some, X}.
  44
  45 -spec map(t(A), fun((A) -> (B))) ->
  46     t(B).
  47 map({some, X}, F) -> {some, F(X)};
  48 map(none     , _) -> none.
  49
  50 -spec iter(t(A), fun((A) -> (ok))) ->
  51     ok.
  52 iter({some, X}, F) -> ok = F(X);
  53 iter(none     , _) -> ok.
  54
  55 -spec pipe([fun((A) -> t(B))], A) ->
  56     t(B).
  57 pipe([], X) ->
  58     return(X);
  59 pipe([F|Fs], X) ->
  60     case F(X)
  61     of  none      -> none
  62     ;   {some, Y} -> pipe(Fs, Y)
  63     end.
  64
  65 -spec of_result(hope_result:t(A, _B)) ->
  66     t(A).
  67 of_result({ok, X})    -> {some, X};
  68 of_result({error, _}) -> none.
  69
  70 -spec of_undefined(undefined | A) ->
  71     t(A).
  72 of_undefined(undefined) -> none;
  73 of_undefined(X)         -> {some, X}.