Relude
relude
is a custom prelude, an alternative to default Prelude
.
relude
tries to achieve the following goals:
-
Avoid all partial functions
(like head :: [a] -> a
). The types of partial functions lie about their
behavior and usage of such functions can lead to the unexpected bugs. Though
you can still use some unsafe functions from Relude.Unsafe
module, but they
are not exported by default.
-
Type-safety. We like to make invalid states unrepresantable. And if it's
possible to express this concept through the types then we will do it.
Example:
whenNotNull :: Applicative f => [a] -> (NonEmpty a -> f ()) -> f ()
instead of
whenNotNull :: Applicative f => [a] -> ([a] -> f ()) -> f ()
-
Performance. Prefer Text
over String
,
use spaceleak-free functions (like our custom sum
and product
), introduce
{-# INLINE #-}
and {-# SPECIALIZE #-}
pragmas where appropriate.
-
Minimalism (low number of dependencies). We don't force users of relude
to
stick to some specific lens or text formatting or logging library.
-
Convenience (like lifted to MonadIO
functions, more reexports). But we
want to bring common types and functions (like containers
and bytestrng
)
into scope because they are used in almost every application anyways.
-
Provide excellent documentation.
-
User-friendliness. Ability to quickly migrate to relude
if you're familiar
with the common libraries like text
and containers
.
-
Exploration. Experiment with new ideas and proposals without introducing
breaking changes. relude
uses the approach with Extra.*
modules which are not
exported by default so it's quite easy to bring something new and let users
decide to use it or not.
This README contains introduction to Relude
and a tutorial on how to use it.
Structure of this tutorial
This tutorial has several parts:
- Get started.
- Difference from
Prelude
.
- Reexports.
- What's new?
- Migration guide.
This is neither a tutorial on Haskell nor tutorial on each function contained
in Relude
. For detailed documentation of every function together with examples
and usage, see Haddock documentation.
Get started ↑
If you want to start using relude
in your project and explore it with the help
of compiler, set everything up according to the instructions below.
base-noprelude
This is the recommended way to use custom prelude. It requires you to perform
the following steps:
- Replace
base
dependency with corresponding version of base-noprelude
in
your .cabal
file.
- Add the following
Prelude
module to your project (both to filesystem and to exposed-modules
):
module Prelude
( module Relude
) where
import Relude
NOTE: if you use summoner
to generate Haskell project,
this tool can automatically create such structure for you when you specify custom prelude.
- Optionally modify your
Prelude
to include more or less functions. Probably
you want to hide something from Relude
module. Or maybe you want to add
something from Relude.Extra.*
modules!
This is a very convenient way to add a custom prelude to your project because
you don't need to import module manually inside each file and enable the
NoImplicitPrelude
extension.
Per-file configuration
Disable the built-in prelude at the top of your file:
{-# LANGUAGE NoImplicitPrelude #-}
Or directly in your project .cabal
file, if you want to use in every module by default:
default-extensions: NoImplicitPrelude
Then add the following import to your modules:
import Relude
Difference from Prelude ↑
head
, tail
, last
, init
work with NonEmpty a
instead of [a]
.
undefined
triggers a compiler warning, because you probably don't want to
leave undefined
in your code. Either use throwIO
, Except
, error
or
bug
.
- Multiple sorting functions are available without imports:
sortBy :: (a -> a -> Ordering) -> [a] -> [a]
: sorts list using given custom comparator.
sortWith :: Ord b => (a -> b) -> [a] -> [a]
: sorts a list based on some property of its elements.
sortOn :: Ord b => (a -> b) -> [a] -> [a]
: just like sortWith
, but more
time-efficient if function is calculated slowly (though less
space-efficient). So you should write sortOn length
(would sort elements
by length) but sortWith fst
(would sort list of pairs by first element).
- Functions
sum
and product
are strict now, which makes them more efficient.
- If you try to do something like
putStrLn "hi"
, you'll get an error message if
OverloadedStrings
is enabled – it happens because the compiler doesn't know what
type to infer for the string. Use putTextLn
in this case.
- Since
show
doesn't come from Show
anymore, you need to export Show
from
Text.Show
module if you want to implement Show
instance manually.
- You can't call
elem
and notElem
functions over Set
and HashSet
. These
functions are forbidden for these two types because of the performance reasons.
error
takes Text
.
lookup
for lists is not exported.
Reexports ↑
relude
reexports some parts of the following libraries:
base
containers
unordered-containers
text
bytestring
transformers
mtl
deepseq
stm
If you want to clean up imports after switching to relude
, you can use
relude
-specific .hlint.yaml
configuration for this task.
base
Some generally useful modules from base
package, like: Control.Applicative
,
Data.Traversable
, Data.Monoid
, Data.List
, and lots of others.
liftIO
and MonadIO
are exported by default. A lot of IO
functions are
generalized to MonadIO
.
Bifunctor
type class with useful instances is exported.
first
and second
functions apply a function to first/second part of a tuple (for tuples).
bimap
takes two functions and applies them to first and second parts respectively.
trace
, traceM
, traceShow
, etc. are available by default. GHC will warn you
if you accidentally leave them in code, however (same for undefined
).
We also have data Undefined = Undefined
(which, too, comes with warnings).
relude
reexports Exception
type from the base
package and introduces the
bug
function as an alternative to error
. There's also a very convenient
Exc
pattern-synonym to handle exceptions of different types.
See Relude.Exception
module for details on exceptions.
containers & unordered-containers
The following types from these two packages are exported:
Then, some commonly used types:
- Maps: strict versions of
Map
, HashMap
, IntMap
.
- Sets:
Set
, HashSet
, IntSet
.
- Sequences:
Seq
.
text & bytestring
relude
exports Text
and ByteString
(as well as synonyms LText
and LByteString
for lazy versions) and some functions work with Text
instead of String
–
specifically, IO functions (readFile
, putStrLn
, etc) and show
. In fact, show
is polymorphic and can produce strict or lazy Text
, String
, or ByteString
.
Also, toText/toLText/toString
can convert Text|LText|String
types to
Text/LText/String
. If you want to convert to and from ByteString
use
encodeUtf8/decodeUtf8
functions.
The following parts of these two libraries are exported:
- Transformers:
State[T]
, Reader[T]
, ExceptT
, MaybeT
.
- Classes:
MonadReader
, MonadState
, MonadError
.
Deepseq
deepseq
is exported. For instance, if you want to force deep evaluation of
some value (in IO
), you can write evaluateNF a
. WHNF evaluation is possible
with evaluateWHNF a
.
What's new? ↑
Finally, we can move to part describing the new cool features we bring with relude
.
Available by default
-
Safe analogue for list functions: use viaNonEmpty
function to get Maybe a
.
viaNonEmpty head :: [a] -> Maybe a
-
uncons
splits a list at the first element.
-
ordNub
and sortNub
are O(n log n) versions of nub
(which is quadratic)
and hashNub
and unstableNub
are almost O(n) versions of nub
.
-
(&)
– reverse application. x & f & g
instead of g $ f $ x
is useful sometimes.
-
whenM
, unlessM
, ifM
, guardM
are available and do what you expect
them to do (e.g. whenM (doesFileExist "foo")
).
-
General fold functions:
foldMapA :: (Monoid b, Applicative m, Foldable f) => (a -> m b) -> f a -> m b
foldMapM :: (Monoid b, Monad m, Foldable f) => (a -> m b) -> f a -> m b
-
readMaybe
and readEither
are like read
but total and give either
Maybe
or Either
with parse error.
-
when(Just|Nothing|Left|Right|NotEmpty)[M][_]
let you conditionally execute something. Before:
case mbX of
Nothing -> return ()
Just x -> f x
After:
whenJust mbX $ \x ->
f x
-
for_
for loops. There's also forM_
but for_
looks a bit nicer.
for_ [1..10] $ \i -> do
...
-
andM
, allM
, anyM
, orM
are monadic version of corresponding functions from base
.
-
Conversions between Either
and Maybe
like rightToMaybe
and maybeToLeft
with clear semantic.
-
using(Reader|State)[T]
functions as aliases for flip run(Reader|State)[T]
.
-
One
type class
for creating singleton containers. Even monomorhpic ones like Text
.
-
StaticMap
and DynamicMap
type classes as a
general interface for Map
-like data structures.
-
evaluateWHNF
and evaluateNF
functions as clearer and lifted aliases for
evaluate
and evaluate . force
.
-
MonadFail
instance for Either
.
Need to import explicitly
-
Convenient functions to work with (Bounded a, Enum a)
types:
universe :: (Bounded a, Enum a) => [a
: get all values of the type.
inverseMap :: (Bounded a, Enum a, Ord k) => (a -> k) -> k -> Maybe a
: convert functions like show
to parsers.
-
Nice helpers to deal with newtype
s in a more pleasant way:
ghci> newtype Foo = Foo Bool deriving Show
ghci> under not (Foo True)
Foo False
-
Functions to operate with CallStack
:
>>> foo :: HasCallStack => String; foo = ownName
>>> foo
"foo"
-
A lot of other cool things:
Migration guide ↑
In order to replace default Prelude
with relude
you should start with instructions given in
get started section.
This section describes what you need to change to make your code compile with relude
.
-
Enable -XOverloadedStrings
extension by default for your project.
-
Since head
, tail
, last
and init
work for NonEmpty
you should
refactor your code in one of the multiple ways described below:
- Change
[a]
to NonEmpty a
where it makes sense.
- Use functions which return
Maybe
. There is the viaNonEmpty
function for this.
And you can use it like viaNonEmpty last l
.
tail
is drop 1
. It's almost never a good idea to use tail
from Prelude
.
- Add
import qualified Relude.Unsafe as Unsafe
and replace function with qualified usage.
-
If you use fromJust
or !!
you should use them from import qualified Relude.Unsafe as Unsafe
.
-
If you use foldr
or forM_
or similar for something like Maybe a
or
Either a b
it's recommended to replace usages of such function with
monomorhpic alternatives:
-
Maybe
(?:) :: Maybe a -> a -> a
fromMaybe :: a -> Maybe a -> a
maybeToList :: Maybe a -> [a]
maybeToMonoid :: Monoid m => Maybe m -> m
maybe :: b -> (a -> b) -> Maybe a -> b
whenJust :: Applicative f => Maybe a -> (a -> f ()) -> f ()
whenJustM :: Monad m => m (Maybe a) -> (a -> m ()) -> m ()
-
Either
fromLeft :: a -> Either a b -> a
fromRight :: b -> Either a b -> b
either :: (a -> c) -> (b -> c) -> Either a b -> c
whenRight_ :: Applicative f => Either l r -> (r -> f ()) -> f ()
whenRightM_ :: Monad m => m (Either l r) -> (r -> m ()) -> m ()
-
Forget about String
type.
- Replace
putStr
and putStrLn
with putText
and putTextLn
.
- Replace
(++)
with (<>)
for String
-like types.
- Try to use
fmt
library if you need to construct messages.
- Use
toText/toLText/toString
functions to convert to Text/LazyText/String
types.
- Use
encodeUtf8/decodeUtf8
to convert to/from ByteString
.
-
Run hlint
using .hlint.yaml
file from relude
package to cleanup code and imports.
Acknowledgement
Icons made by Freepik from www.flaticon.com is licensed by CC 3.0 BY.