Implement hope_result:(map_error and tag_error)

[hope.git] / src / hope_result.erl

diff --git a/src/hope_result.erl b/src/hope_result.erl
index feda537..dec6e79 100644 (file)
--- a/src/hope_result.erl
+++ b/src/hope_result.erl
@@ -1,12 +1,17 @@
 -module(hope_result).
 
+-behavior(hope_gen_monad).
 
 -export_type(
     [ t/2
     ]).
 
 -export(
-    [ pipe/2
+    [ return/1
+    , map/2
+    , map_error/2
+    , tag_error/2
+    , pipe/2
     , lift_exn/1
     , lift_exn/2
     ]).
@@ -18,6 +23,32 @@
     .
 
 
+-spec return(A) ->
+    {ok, A}.
+return(X) ->
+    {ok, X}.
+
+-spec map(t(A, Error), fun((A) -> (B))) ->
+    t(B, Error).
+map({ok, X}, F) ->
+    {ok, F(X)};
+map({error, _}=Error, _) ->
+    Error.
+
+-spec map_error(t(A, B), fun((B) -> (C))) ->
+    t(A, C).
+map_error({ok, _}=Ok, _) ->
+    Ok;
+map_error({error, Reason}, F) ->
+    {error, F(Reason)}.
+
+-spec tag_error(t(A, Reason), Tag) ->
+    t(A, {Tag, Reason}).
+tag_error({ok, _}=Ok, _) ->
+    Ok;
+tag_error({error, Reason}, Tag) ->
+    {error, {Tag, Reason}}.
+
 -spec pipe([F], X) ->
     t(Ok, Error)
     when X     :: any()
@@ -61,6 +92,6 @@ lift_exn(F, Label) when is_function(F, 1) ->
         try
             {ok, F(X)}
         catch Class:Reason ->
-            {error, {Label, {Class, Reason}}}
+            tag_error({error, {Class, Reason}}, Label)
         end
     end.