src/hope_option.erl

   1 -module(hope_option).
   2
   3 -behavior(hope_gen_monad).
   4
   5 -export_type(
   6     [ t/1
   7     ]).
   8
   9 -export(
  10     % Generic monad interface
  11     [ return/1
  12     , map/2
  13     , pipe/2
  14
  15     % Specific to hope_option:t()
  16     , return/2
  17     , put/2
  18     , get/2
  19     , iter/2
  20     , of_result/1
  21     , of_undefined/1
  22     , to_undefined/1
  23     , validate/2
  24     ]).
  25
  26
  27 -type t(A) ::
  28       none
  29     | {some, A}
  30     .
  31
  32
  33 -spec put(A, fun((A) -> boolean())) ->
  34     t(A).
  35 put(X, F) ->
  36     return(X, F).
  37
  38 -spec get(t(A), Default :: A) ->
  39     A.
  40 get({some, X}, _) -> X;
  41 get(none     , Y) -> Y.
  42
  43 -spec return(A) ->
  44     {some, A}.
  45 return(X) ->
  46     {some, X}.
  47
  48 -spec return(A, fun((A) -> boolean())) ->
  49     t(A).
  50 return(X, Condition) ->
  51     case Condition(X)
  52     of  true  -> {some, X}
  53     ;   false -> none
  54     end.
  55
  56 -spec map(t(A), fun((A) -> (B))) ->
  57     t(B).
  58 map({some, X}, F) -> {some, F(X)};
  59 map(none     , _) -> none.
  60
  61 -spec iter(t(A), fun((A) -> (any()))) ->
  62     {}.
  63 iter({some, X}, F) ->
  64     _ = F(X),
  65     {};
  66 iter(none, _) ->
  67     {}.
  68
  69 -spec pipe([fun((A) -> t(B))], A) ->
  70     t(B).
  71 pipe([], X) ->
  72     return(X);
  73 pipe([F|Fs], X) ->
  74     case F(X)
  75     of  none      -> none
  76     ;   {some, Y} -> pipe(Fs, Y)
  77     end.
  78
  79 -spec of_result(hope_result:t(A, _B)) ->
  80     t(A).
  81 of_result({ok, X})    -> {some, X};
  82 of_result({error, _}) -> none.
  83
  84 -spec of_undefined(undefined | A) ->
  85     t(A).
  86 of_undefined(undefined) -> none;
  87 of_undefined(X)         -> {some, X}.
  88
  89 -spec to_undefined(t(A)) ->
  90     undefined | A.
  91 to_undefined(none)      -> undefined;
  92 to_undefined({some, X}) -> X.
  93
  94 -spec validate(t(A), fun((A) -> boolean())) ->
  95     t(A).
  96 validate(none, _) ->
  97     none;
  98 validate({some, X}=T, F) ->
  99     case F(X)
 100     of  false -> none
 101     ;   true  -> T
 102     end.