Fea/do operator

pyresult/__init__.py

@@ -16,6 +16,7 @@
     and_then,
     and_else,
     resolve,
+    do,
 )
 
 __author__ = """Jindrich Kralevic Smitka"""

pyresult/operators.py

@@ -60,15 +60,15 @@ def fold(res):
     fold: (List Result a x) -> Result (List a) (List x)
     '''
     len_res = len(res)
-    val = [None]*len_res
-    err = [None]*len_res
+    val = [None] * len_res
+    err = [None] * len_res
 
-    for i, e in enumerate(res):
-        e = result(e)
-        if is_ok(e):
-            val[i] = e.value
+    for i, item in enumerate(res):
+        result_item = result(item)
+        if is_ok(result_item):
+            val[i] = result_item.value
         else:
-            err[i] = e.value
+            err[i] = result_item.value
 
     if None in val:
         return error(err)
@@ -83,3 +83,15 @@ def resolve(res):
     '''
     res = result(res)
     return res if is_error(res) else result(res.value)
+
+
+def do(func, res):  # pylint: disable=invalid-name
+    '''Run `func` when result `res` is ok
+    and then return `res` if function return ok result,
+    otherwise return its error result.
+
+    do: (val -> Result x e) -> Result val e -> Result val e
+    '''
+    res = result(res)
+
+    return and_then(lambda _: res, func(res.value)) if is_ok(res) else res

tests/test_operators.py

@@ -9,7 +9,8 @@
     and_then,
     and_else,
     fold,
-    resolve
+    resolve,
+    do
 )
 
 
@@ -113,3 +114,21 @@ def test_resolve_isnt_ok():
     val = (OK, (OK, 'val'))
 
     assert resolve(val) == (OK, 'val')
+
+
+def test_do_return_original_error():
+    err = error('ERROR')
+
+    assert do(ok, err) == err
+
+
+def test_do_return_original_result():
+    result = ok('FOO BAR')
+
+    assert do(lambda _: ok('BAZ FOO'), result) == result
+
+
+def test_do_return_error_from_function():
+    err = error('BAZ FOO')
+
+    assert do(lambda _: err, ok('FOO BAR')) == err