elemIndex -package:foldl

elemIndex :: Eq a => a -> [a] -> Maybe Int
base Data.List GHC.OldList
The elemIndex function returns the index of the first element in the given list which is equal (by ==) to the query element, or Nothing if there is no such element. For the result to be Nothing, the list must be finite. 
>>> elemIndex 4 [0..]
Just 4
elemIndex :: Word8 -> ByteString -> Maybe Int
bytestring Data.ByteString, rio RIO.ByteString
O(n) The elemIndex function returns the index of the first element in the given ByteString which is equal to the query element, or Nothing if there is no such element. This implementation uses memchr(3).
elemIndex :: Char -> ByteString -> Maybe Int
bytestring Data.ByteString.Char8
O(n) The elemIndex function returns the index of the first element in the given ByteString which is equal (by memchr) to the query element, or Nothing if there is no such element.
elemIndex :: Word8 -> ByteString -> Maybe Int64
bytestring Data.ByteString.Lazy, rio RIO.ByteString.Lazy
O(n) The elemIndex function returns the index of the first element in the given ByteString which is equal to the query element, or Nothing if there is no such element. This implementation uses memchr(3).
elemIndex :: Char -> ByteString -> Maybe Int64
bytestring Data.ByteString.Lazy.Char8
O(n) The elemIndex function returns the index of the first element in the given ByteString which is equal (by memchr) to the query element, or Nothing if there is no such element.
elemIndex :: Word8 -> ShortByteString -> Maybe Int
bytestring Data.ByteString.Short Data.ByteString.Short.Internal, filepath System.OsPath.Data.ByteString.Short
O(n) The elemIndex function returns the index of the first element in the given ShortByteString which is equal to the query element, or Nothing if there is no such element.
elemIndex :: Word16 -> ShortByteString -> Maybe Int
filepath System.OsPath.Data.ByteString.Short.Word16
O(n) The elemIndex function returns the index of the first element in the given ShortByteString which is equal to the query element, or Nothing if there is no such element.
elemIndex :: Eq a => a -> Vector a -> Maybe Int
vector Data.Vector
O(n) Yield Just the index of the first occurrence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Vector v a, Eq a) => a -> v a -> Maybe Int
vector Data.Vector.Generic
O(n) Yield Just the index of the first occurrence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Prim a, Eq a) => a -> Vector a -> Maybe Int
vector Data.Vector.Primitive
O(n) Yield Just the index of the first occurrence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Storable a, Eq a) => a -> Vector a -> Maybe Int
vector Data.Vector.Storable
O(n) Yield Just the index of the first occurrence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Unbox a, Eq a) => a -> Vector a -> Maybe Int
vector Data.Vector.Unboxed
O(n) Yield Just the index of the first occurrence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: UTF8Bytes b s => Word8 -> b -> Maybe s
utf8-string Codec.Binary.UTF8.Generic
elemIndex :: Eq a => a -> [a] -> Maybe Int
rio RIO.List, base-prelude BasePrelude
The elemIndex function returns the index of the first element in the given list which is equal (by ==) to the query element, or Nothing if there is no such element. 
>>> elemIndex 4 [0..]
Just 4
elemIndex :: (Vector v a, Eq a) => a -> v a -> Maybe Int
rio RIO.Vector
O(n) Yield Just the index of the first occurence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: Eq a => a -> Vector a -> Maybe Int
rio RIO.Vector.Boxed
O(n) Yield Just the index of the first occurence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Storable a, Eq a) => a -> Vector a -> Maybe Int
rio RIO.Vector.Storable
O(n) Yield Just the index of the first occurence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Unbox a, Eq a) => a -> Vector a -> Maybe Int
rio RIO.Vector.Unboxed
O(n) Yield Just the index of the first occurence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Monad m, Num n) => Word8 -> Producer ByteString m () -> m (Maybe n)
pipes-bytestring Pipes.ByteString
Find the index of an element that matches the given Word8
elemIndex :: (ListLike full item, Eq item) => item -> full -> Maybe Int
ListLike Data.ListLike Data.ListLike.Base
Returns the index of the element, if it exists.
elemIndex :: (C n, Eq a) => a -> [a] -> Maybe n
numeric-prelude NumericPrelude.List.Generic
elemIndex :: (Monad m, Eq a) => a -> SerialT m a -> m (Maybe Int)
streamly Streamly.Internal.Data.Stream.IsStream Streamly.Internal.Data.Stream.IsStream.Eliminate Streamly.Prelude
Returns the first index where a given value is found in the stream. 
elemIndex a = Stream.findIndex (== a)
elemIndex :: (Storable a, Eq a) => a -> Vector a -> Maybe Int
storablevector Data.StorableVector
O(n) The elemIndex function returns the index of the first element in the given Vector which is equal to the query element, or Nothing if there is no such element.
elemIndex :: (Vector v a, Eq a) => a -> Vector v n a -> Maybe (Finite n)
vector-sized Data.Vector.Generic.Sized
O(n) Yield Just the index of the first occurence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.
elemIndex :: (Prim a, Eq a) => a -> Vector n a -> Maybe (Finite n)
vector-sized Data.Vector.Primitive.Sized
O(n) Yield Just the index of the first occurence of the given element or Nothing if the vector does not contain the element. This is a specialised version of findIndex.