Add map(), find() and next() methods to AsyncIter (#3921)

* properly handle prefixes

* Docsss and typehinting

* aaaaaaaaaaa

* Apply suggestions from code review

* ffs

* docs

* docs

* Apply suggestions from code review

Co-authored-by: jack1142 <6032823+jack1142@users.noreply.github.com>

* skip await if map is none

* implement `.next()`

Co-authored-by: jack1142 <6032823+jack1142@users.noreply.github.com>

redbot/core/utils/__init__.py

@@ -17,15 +17,17 @@ from typing import (
     Tuple,
     TypeVar,
     Union,
-    Set,
-    TYPE_CHECKING,
     Generator,
+    Coroutine,
 )
 
+from discord.utils import maybe_coroutine
+
 __all__ = ("bounded_gather", "bounded_gather_iter", "deduplicate_iterables", "AsyncIter")
 
-_T = TypeVar("_T")
 
+_T = TypeVar("_T")
+_S = TypeVar("_S")
 
 # Benchmarked to be the fastest method.
 def deduplicate_iterables(*iterables):
@@ -297,6 +299,7 @@ class AsyncIter(AsyncIterator[_T], Awaitable[List[_T]]):  # pylint: disable=dupl
         self._iterator = iter(iterable)
         self._i = 0
         self._steps = steps
+        self._map = None
 
     def __aiter__(self) -> AsyncIter[_T]:
         return self
@@ -310,7 +313,7 @@ class AsyncIter(AsyncIterator[_T], Awaitable[List[_T]]):  # pylint: disable=dupl
             self._i = 0
             await asyncio.sleep(self._delay)
         self._i += 1
-        return item
+        return await maybe_coroutine(self._map, item) if self._map is not None else item
 
     def __await__(self) -> Generator[Any, None, List[_T]]:
         """Returns a list of the iterable.
@@ -325,6 +328,37 @@ class AsyncIter(AsyncIterator[_T], Awaitable[List[_T]]):  # pylint: disable=dupl
         """
         return self.flatten().__await__()
 
+    async def next(self, default: Any = ...) -> _T:
+        """Returns a next entry of the iterable.
+
+        Parameters
+        ----------
+        default: Optional[Any]
+            The value to return if the iterator is exhausted.
+
+        Raises
+        ------
+        StopAsyncIteration
+            When ``default`` is not specified and the iterator has been exhausted.
+
+        Examples
+        --------
+        >>> from redbot.core.utils import AsyncIter
+        >>> iterator = AsyncIter(range(5))
+        >>> await iterator.next()
+        0
+        >>> await iterator.next()
+        1
+
+        """
+        try:
+            value = await self.__anext__()
+        except StopAsyncIteration:
+            if default is ...:
+                raise
+            value = default
+        return value
+
     async def flatten(self) -> List[_T]:
         """Returns a list of the iterable.
 
@@ -339,8 +373,7 @@ class AsyncIter(AsyncIterator[_T], Awaitable[List[_T]]):  # pylint: disable=dupl
         return [item async for item in self]
 
     def filter(self, function: Callable[[_T], Union[bool, Awaitable[bool]]]) -> AsyncFilter[_T]:
-        """
-        Filter the iterable with an (optionally async) predicate.
+        """Filter the iterable with an (optionally async) predicate.
 
         Parameters
         ----------
@@ -424,3 +457,69 @@ class AsyncIter(AsyncIterator[_T], Awaitable[List[_T]]):  # pylint: disable=dupl
                 yield item
                 _temp.add(item)
         del _temp
+
+    async def find(
+        self,
+        predicate: Callable[[_T], Union[bool, Awaitable[bool]]],
+        default: Optional[Any] = None,
+    ) -> AsyncIterator[_T]:
+        """Calls ``predicate`` over items in iterable and return first value to match.
+
+        Parameters
+        ----------
+        predicate: Union[Callable, Coroutine]
+            A function that returns a boolean-like result. The predicate provided can be a coroutine.
+        default: Optional[Any]
+            The value to return if there are no matches.
+
+        Raises
+        ------
+        TypeError
+            When ``predicate`` is not a callable.
+
+        Examples
+        --------
+        >>> from redbot.core.utils import AsyncIter
+        >>> await AsyncIter(range(3)).find(lambda x: x == 1)
+        1
+        """
+        while True:
+            try:
+                elem = await self.__anext__()
+            except StopAsyncIteration:
+                return default
+            ret = await maybe_coroutine(predicate, elem)
+            if ret:
+                return elem
+
+    def map(self, func: Callable[[_T], Union[_S, Awaitable[_S]]]) -> AsyncIter[_S]:
+        """Set the mapping callable for this instance of `AsyncIter`.
+
+        .. important::
+            This should be called after AsyncIter initialization and before any other of its methods.
+
+        Parameters
+        ----------
+        func: Union[Callable, Coroutine]
+            The function to map values to. The function provided can be a coroutine.
+
+        Raises
+        ------
+        TypeError
+            When ``func`` is not a callable.
+
+        Examples
+        --------
+        >>> from redbot.core.utils import AsyncIter
+        >>> async for value in AsyncIter(range(3)).map(bool):
+        ...     print(value)
+        False
+        True
+        True
+
+        """
+
+        if not callable(func):
+            raise TypeError("Mapping must be a callable.")
+        self._map = func
+        return self