Pretty self-explanatory. I know that makeClassy should create typeclasses, but I see no difference between the two.
PS. Bonus points for explaining the default behaviour of both.
Note: This answer is based on lens 4.4 or newer. There were some changes to the TH in that version, so I don't know how much of it applies to older versions of lens.
Organization of the lens TH functions
The lens TH functions are all based on one function, makeLensesWith (also named makeFieldOptics inside lens). This function takes a LensRules argument, which describes exactly what is generated and how.
So to compare makeLenses and makeFields, we only need to compare the LensRules that they use. You can find them by looking at the source:
makeLenses
lensRules :: LensRules
lensRules = LensRules
{ _simpleLenses = False
, _generateSigs = True
, _generateClasses = False
, _allowIsos = True
, _classyLenses = const Nothing
, _fieldToDef = \_ n ->
case nameBase n of
'_':x:xs -> [TopName (mkName (toLower x:xs))]
_ -> []
}
makeFields
defaultFieldRules :: LensRules
defaultFieldRules = LensRules
{ _simpleLenses = True
, _generateSigs = True
, _generateClasses = True -- classes will still be skipped if they already exist
, _allowIsos = False -- generating Isos would hinder field class reuse
, _classyLenses = const Nothing
, _fieldToDef = camelCaseNamer
}
What do these mean?
Now we know that the differences are in the simpleLenses, generateClasses, allowIsos and fieldToDef options. But what do those options actually mean?
makeFields will never generate type-changing optics. This is controlled by the simpleLenses = True option. That option doesn't have haddocks in the current version of lens. However, lens HEAD added documentation for it:
-- | Generate "simple" optics even when type-changing optics are possible.
-- (e.g. 'Lens'' instead of 'Lens')
So makeFields will never generate type-changing optics, while makeLenses will if possible.
makeFields will generate classes for the fields. So for each field foo, we have a class:
class HasFoo t where
foo :: Lens' t <Type of foo field>
This is controlled by the generateClasses option.
makeFields will never generate Iso's, even if that would be possible (controlled by the allowIsos option, which doesn't seem to be exported from Control.Lens.TH)
While makeLenses simply generates a top-level lens for each field that starts with an underscore (lowercasing the first letter after the underscore), makeFields will instead generate instances for the HasFoo classes. It also uses a different naming scheme, explained in a comment in the source code:
-- | Field rules for fields in the form # prefixFieldname or _prefixFieldname #
-- If you want all fields to be lensed, then there is no reason to use an #_# before the prefix.
-- If any of the record fields leads with an #_# then it is assume a field without an #_# should not have a lens created.
camelCaseFields :: LensRules
camelCaseFields = defaultFieldRules
So makeFields also expect that all fields are not just prefixed with an underscore, but also include the data type name as a prefix (as in data Foo = { _fooBar :: Int, _fooBaz :: Bool }). If you want to generate lenses for all fields, you can leave out the underscore.
This is all controlled by the _fieldToDef (exported as lensField by Control.Lens.TH).
As you can see, the Control.Lens.TH module is very flexible. Using makeLensesWith, you can create your very own LensRules if you need a pattern not covered by the standard functions.
Disclaimer: this is based on experimenting with the working code; it gave me enough information to proceed with my project, but I'd still prefer a better-documented answer.
data Stuff = Stuff {
_foo
_FooBar
_stuffBaz
}
makeLenses
Will create foo as a lens accessor to Stuff
Will create fooBar (changing the capitalized name to lowercase);
makeFields
Will create baz and a class HasBaz; it will make Stuff an instance of that class.
Normal
makeLenses creates a single top-level optic for each field in the type. It looks for fields that start with an underscore (_) and it creates an optic that is as general as possible for that field.
If your type has one constructor and one field you'll get an Iso.
If your type has one constructor and multiple fields you'll get many Lens.
If your type has multiple constructors you'll get many Traversal.
Classy
makeClassy creates a single class containing all the optics for your type. This version is used to make it easy to embed your type in another larger type achieving a kind of subtyping. Lens and Traversal optics will be created according to the rules above (Iso is excluded because it hinders the subtyping behavior.)
In addition to one method in the class per field you'll get an extra method that makes it easy to derive instances of this class for other types. All of the other methods have default instances in terms of the top-level method.
data T = MkT { _field1 :: Int, _field2 :: Char }
class HasT a where
t :: Lens' a T
field1 :: Lens' a Int
field2 :: Lens' a Char
field1 = t . field1
field2 = t . field2
instance HasT T where
t = id
field1 f (MkT x y) = fmap (\x' -> MkT x' y) (f x)
field2 f (MkT x y) = fmap (\y' -> MkT x y') (f y)
data U = MkU { _subt :: T, _field3 :: Bool }
instance HasT U where
t f (MkU x y) = fmap (\x' -> MkU x' y) (f x)
-- field1 and field2 automatically defined
This has the additional benefit that it is easy to export/import all the lenses for a given type. import Module (HasT(..))
Fields
makeFields creates a single class per field which is intended to be reused between all types that have a field with the given name. This is more of a solution to record field names not being able to be shared between types.
Related
I'm trying to learn Haskell.
I'm reading the code on here[1]. I just copy and past some part of the code from lines:46 and 298-300.
Question: What does (..) mean?
I Hoggled it but I got no result.
module Pos.Core.Types(
-- something here
SharedSeed (..) -- what does this mean?
) where
newtype SharedSeed = SharedSeed
{ getSharedSeed :: ByteString
} deriving (Show, Eq, Ord, Generic, NFData, Typeable)
[1] https://github.com/input-output-hk/cardano-sl/blob/master/core/Pos/Core/Types.hs
The syntax of import/export lists has not much to do with the syntax of Haskell itself. It's just a comma-separated listing of everything you want to export from your module. Now, there's a problem there because Haskell really has two languages with symbols that may have the same name. This is particularly common with newtypes like the one in your example: you have a type-level name SharedSeed :: *, and also a value-level name (data constructor) SharedSeed :: ByteString -> SharedSeed.
This only happens with uppercase names, because lowercase at type level are always local type variables. Thus the convention in export lists that uppercase names refer to types.
But just exporting the type does not allow users to actually construct values of that type. That's often prudent: not all internal-representation values might make legal values of the newtype (see Bartek's example), so then it's better to only export a safe smart constructor instead of the unsafe data constructor.
But other times, you do want to make the data constructor available, certainly for multi-constructor types like Maybe. To that end, export lists have three syntaxes you can use:
module Pos.Core.Types(
SharedSeed (SharedSeed) will export the constructor SharedSeed. In this case that's of course the only constructor anyway, but if there were other constructors they would not be exported with this syntax.
SharedSeed (..) will export all constructors. Again, in this case that's only SharedSeed, but for e.g. Maybe it would export both Nothing and Just.
pattern SharedSeed will export ShareSeed as a standalone pattern (independent of the export of the ShareSeed type). This requires the -XPatternSynonyms extension.
)
It means "export all constructors and record fields for this data type".
When writing a module export list, there's three4 ways you can export a data type:
module ModuleD(
D, -- just the type, no constructors
D(..), -- the type and all its constructors
D(DA) -- the type and the specific constructor
) where
data D = DA A | DB B
If you don't export any constructors, the type, well, can't be constructed, at least directly. This is useful if you e.g. want to enforce some runtime invariants on the data type:
module Even (evenInt, toInt) where
newtype EvenInt = EvenInt Int deriving (Show, Eq)
evenInt :: Int -> Maybe EvenInt
evenInt x = if x `mod` 2 == 0 then Just x else Nothing
toInt :: EvenInt -> Int
toInt (EvenInt x) = x
The caller code can now use this type, but only in the allowed manner:
x = evenInt 2
putStrLn $ if isJust x then show . toInt . fromJust $ x else "Not even!"
As a side note, toInt is usually implemented indirectly via the record syntax for convenience:
data EvenInt = EvenInt { toInt :: Int }
4 See #leftaroundabout's answer
I have several datatypes representing the state of an application. In various places in the datatype, I have embedded functions or monadic actions, eg.
data Foo = Foo Int (ActionM String)
data Bar = Bar Foo (Maybe Bar) (ActionM ())
I need to encode most of these datatypes as json so I can send it to the browser for display. Using deriveJSON (from the Aeson package) doesn't work because instances for ActionM can't be derived. However, I don't actually want those bits to be sent anyway. I currently have an approach which works but is basically copy-pasting the full set of datatypes and manually removing the embeded ActionM fields.
I (think I) need one of a couple of things. Either
a way to tell deriveJSON to just ignore fields that it can't figure out, and maybe parse them back into undefined. As far as I can tell this doesn't exist
a way to automatically generate a parallel set of datatypes with these fields removed. So I want to write something like
applyMagic Bar
and get back
data Foo' = Foo' Int
data Bar' = Bar' Foo' (Maybe Bar')
Is any of this possible, and how would I do it?
This is a simplistic solution, but couldn't you do something like
data Foo' = Foo' Int
type Foo = (ActionM String, Foo')
and simply obtain the second element of the tuple when you want to serialize?
Tuples are an instance of ComonadEnv, so you could also use functions like ask and extract.
Edit. Bar is a more complicated case because it is a recursive type. But it could be handled using the CofreeT comonad transformer:
import Data.Functor.Identity
import Data.Bifunctor (second)
import Control.Comonad -- from 'comonad'
import Control.Comonad.Hoist.Class
import Control.Comonad.Trans.Cofree -- from 'free'
-- Orphan ComonadHoist instance that will likely be added in future
-- versions of free
instance Functor f => ComonadHoist (CofreeT f) where
cohoist g = CofreeT . fmap (second (cohoist g)) . g . runCofreeT
type Bar = CofreeT Maybe ((,) (ActionM ())) Foo
type Bar' = Cofree Maybe Foo'
applyMagic :: Bar -> Bar'
applyMagic = cohoist (Identity . extract) . fmap extract
CofreeT Maybe ((,) (ActionM ())) Foo is a non-empty list of Foo values that have been annotated with ActionM () values.
Cofree Maybe Foo' is a non-empty list of Foo' values, without extra annotations (Cofree Maybe Foo is a synonym for CofreeT Maybe Identity Foo', where Identity works as the trivial comonad.).
To transform one into the other, applyMagic first uses fmap extract to transform all the Foos into Foo's, and then uses cohoist from ComonadHoist to remove the "annotation layer" underneath CofreeT.
In general, values with "extra context" can often be modeled with comonads.
I love Lens library and I love how it works, but sometimes it introduces so many problems, that I regret I ever started using it. Lets look at this simple example:
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data Data = A { _x :: String, _y :: String }
| B { _x :: String }
makeLenses ''Data
main = do
let b = B "x"
print $ view y b
it outputs:
""
And now imagine - we've got a datatype and we refactor it - by changing some names. Instead of getting error (in runtime, like with normal accessors) that this name does not longer apply to particular data constructor, lenses use mempty from Monoid to create default object, so we get strange results instead of error. Debugging something like this is almost impossible.
Is there any way to fix this behaviour? I know there are some special operators to get the behaviour I want, but all "normal" looking functions from lenses are just horrible. Should I just override them with my custom module or is there any nicer method?
As a sidenote: I want to be able to read and set the arguments using lens syntax, but just remove the behaviour of automatic result creating when field is missing.
It sounds like you just want to recover the exception behavior. I vaguely recall that this is how view once worked. If so, I expect a reasonable choice was made with the change.
Normally I end up working with (^?) in the cases you are talking about:
> b ^? y
Nothing
If you want the exception behavior you can use ^?!
> b ^?! y
"*** Exception: (^?!): empty Fold
I prefer to use ^? to avoid partial functions and exceptions, similar to how it is commonly advised to stay away from head, last, !! and other partial functions.
Yes, I too have found it a bit odd that view works for Traversals by concatenating the targets. I think this is because of the instance Monoid m => Applicative (Const m). You can write your own view equivalent that doesn't have this behaviour by writing your own Const equivalent that doesn't have this instance.
Perhaps one workaround would be to provide a type signature for y, so know know exactly what it is. If you had this then your "pathological" use of view wouldn't compile.
data Data = A { _x :: String, _y' :: String }
| B { _x :: String }
makeLenses ''Data
y :: Lens' Data String
y = y'
You can do this by defining your own view1 operator. It doesn't exist in the lens package, but it's easy to define locally.
{-# LANGUAGE TemplateHaskell #-}
import Control.Lens
data Data = A { _x :: String, _y :: String }
| B { _x :: String }
makeLenses ''Data
newtype Get a b = Get { unGet :: a }
instance Functor (Get a) where
fmap _ (Get x) = Get x
view1 :: LensLike' (Get a) s a -> s -> a
view1 l = unGet . l Get
works :: Data -> String
works = view1 x
-- fails :: Data -> String
-- fails = view1 y
-- Bug.hs:23:15:
-- No instance for (Control.Applicative.Applicative (Get String))
-- arising from a use of ‘y’
Learning Haskell, I write a formatter of C++ header files. First, I parse all class members into a-collection-of-class-members which is then passed to the formatting routine. To represent class members I have
data ClassMember = CmTypedef Typedef |
CmMethod Method |
CmOperatorOverload OperatorOverload |
CmVariable Variable |
CmFriendClass FriendClass |
CmDestructor Destructor
(I need to classify the class members this way because of some peculiarities of the formatting style.)
The problem that annoys me is that to "drag" any function defined for the class member types to the ClassMember level, I have to write a lot of redundant code. For example,
instance Formattable ClassMember where
format (CmTypedef td) = format td
format (CmMethod m) = format m
format (CmOperatorOverload oo) = format oo
format (CmVariable v) = format v
format (CmFriendClass fc) = format fc
format (CmDestructor d) = format d
instance Prettifyable ClassMember where
-- same story here
On the other hand, I would definitely like to have a list of ClassMember objects (at least, I think so), hence defining it as
data ClassMember a = ClassMember a
instance Formattable ClassMember a
format (ClassMember a) = format a
doesn't seem to be an option.
The alternatives I'm considering are:
Store in ClassMember not object instances themselves, but functions defined on the corresponding types, which are needed by the formatting routine. This approach breaks the modularity, IMO, as the parsing results, represented by [ClassMember], need to be aware of all their usages.
Define ClassMember as an existential type, so [ClassMember] is no longer a problem. I doubt whether this design is strict enough and, again, I need to specify all constraints in the definition, like data ClassMember = forall a . Formattable a => ClassMember a. Also, I would prefer a solution without using extensions.
Is what I'm doing a proper way to do it in Haskell or there is a better way?
First, consider trimming down that ADT a bit. Operator overloads and destructors are special kinds of methods, so it might make more sense to treat all three in CmMethod; Method will then have special ways to separate them. Alternatively, keep all three CmMethod, CmOperatorOverload, and CmDestructor, but let them all contain the same Method type.
But of course, you can reduce the complexity only so much.
As for the specific example of a Show instance: you really don't want to write that yourself except in some special cases. For your case, it's much more reasonable to have the instance derived automatically:
data ClassMember = CmTypedef Typedef
| CmMethod Method
| ...
| CmDestructor Destructor
deriving (Show)
This will give different results from your custom instance – because yours is wrong: showing a contained result should also give information about the constructor.
If you're not really interested in Show but talking about another class C that does something more specific to ClassMembers – well, then you probably shouldn't have defined C in the first place! The purpose of type classes is to express mathematical concepts that hold for a great variety of types.
A possible solution is to use records.
It can be used without extensions and preserves flexibility.
There is still some boilerplate code, but you need to type it only once for all. So if you would need to perform another set of operations over your ClassMember, it would be very easy and quick to do it.
Here is an example for your particular case (template Haskell and Control.Lens makes things easier but are not mandatory):
{-# LANGUAGE TemplateHaskell #-}
module Test.ClassMember
import Control.Lens
-- | The class member as initially defined.
data ClassMember =
CmTypedef Typedef
| CmMethod Method
| CmOperatorOverload OperatorOverload
| CmVariable Variable
| CmFriendClass FriendClass
| CmDestructor Destructor
-- | Some dummy definitions of the data types, so the code will compile.
data Typedef = Typedef
data Method = Method
data OperatorOverload = OperatorOverload
data Variable = Variable
data FriendClass = FriendClass
data Destructor = Destructor
{-|
A data type which defines one function per constructor.
Note the type a, which means that for a given Hanlder "a" all functions
must return "a" (as for a type class!).
-}
data Handler a = Handler
{
_handleType :: Typedef -> a
, _handleMethod :: Method -> a
, _handleOperator :: OperatorOverload -> a
, _handleVariable :: Variable -> a
, _handleFriendClass :: FriendClass -> a
, _handleDestructor :: Destructor -> a
}
{-|
Here I am using lenses. This is not mandatory at all, but makes life easier.
This is also the reason of the TemplateHaskell language pragma above.
-}
makeLenses ''Handler
{-|
A function acting as a dispatcher (the boilerplate code!!!), telling which
function of the handler must be used for a given constructor.
-}
handle :: Handler a -> ClassMember -> a
handle handler member =
case member of
CmTypedef a -> handler^.handleType $ a
CmMethod a -> handler^.handleMethod $ a
CmOperatorOverload a -> handler^.handleOperator $ a
CmVariable a -> handler^.handleVariable $ a
CmFriendClass a -> handler^.handleFriendClass $ a
CmDestructor a) -> handler^.handleDestructor $ a
{-|
A dummy format method.
I kept things simple here, but you could define much more complicated
functions.
You could even define some generic functions separately and... you could define
them with some extra arguments that you would only provide when building
the Handler! An (dummy!) example is the way the destructor function is
constructed.
-}
format :: Handler String
format = Handler
(\x -> "type")
(\x -> "method")
(\x -> "operator")
(\x -> "variable")
(\x -> "Friend")
(destructorFunc $ (++) "format ")
{-|
A dummy function showcasing partial application.
It has one more argument than handleDestructor. In practice you are free
to add as many as you wish as long as it ends with the expected type
(Destructor -> String).
-}
destructorFunc :: (String -> String) -> Destructor -> String
destructorFunc f _ = f "destructor"
{-|
Construction of the pretty handler which illustrates the reason why
using lens by keeping a nice and concise syntax.
The "&" is the backward operator and ".~" is the set operator.
All we do here is to change the functions of the handleType and the
handleDestructor.
-}
pretty :: Handler String
pretty = format & handleType .~ (\x -> "Pretty type")
& handleDestructor .~ (destructorFunc ((++) "Pretty "))
And now we can run some tests:
test1 = handle format (CmDestructor Destructor)
> "format destructor"
test2 = handle pretty (CmDestructor Destructor)
> "Pretty destructor"
I'm working on a basic UI toolkit and am trying to figure out the overall architecture.
I am considering to use WAI's structure for extensibility. A reduced example of the core structure for my UI:
run :: Application -> IO ()
type Application = Event -> UI -> (Picture, UI)
type Middleware = Application -> Application
In WAI, arbitrary values for Middleware are saved in the vault. I think that this is a bad hack to save arbitary values, because it isn't transparent, but I can't think of a sufficient simple structure to replace this vault to give every Middleware a place to save arbitrary values.
I considered to recursively store tuples in tuples:
run :: (Application, x) -> IO ()
type Application = Event -> UI -> (Picture, UI)
type Middleware y x = (Application, x) -> (Application, (y,x))
Or to only use lazy lists to provide a level on which is no need to separate values (which provides more freedom, but also has more problems):
run :: Application -> IO ()
type Application = [Event -> UI -> (Picture, UI)]
type Middleware = Application -> Application
Actually, I would use a modified lazy list solution. Which other solutions might work?
Note that:
I prefer not to use lens at all.
I know UI -> (Picture, UI) could be defined as State UI Picture .
I'm not aware of a solution regarding monads, transformers or FRP. It would be great to see one.
Lenses provide a general way to reference data type fields so that you can extend or refactor your data set without breaking backwards compatibility. I'll use the lens-family and lens-family-th libraries to illustrate this, since they are lighter dependencies than lens.
Let's begin with a simple record with two fields:
{-# LANGUAGE Template Haskell #-}
import Lens.Family2
import Lens.Family2.TH
data Example = Example
{ _int :: Int
, _str :: String
}
makeLenses ''Example
-- This creates these lenses:
int :: Lens' Example Int
str :: Lens' Example String
Now you can write Stateful code that references fields of your data structure. You can use Lens.Family2.State.Strict for this purpose:
import Lens.Family2.State.Strict
-- Everything here also works for `StateT Example IO`
example :: State Example Bool
example = do
s <- use str -- Read the `String`
str .= s ++ "!" -- Set the `String`
int += 2 -- Modify the `Int`
zoom int $ do -- This sub-`do` block has type: `State Int Int`
m <- get
return (m + 1)
The key thing to note is that I can update my data type, and the above code will still compile. Add a new field to Example and everything will still work:
data Example = Example
{ _int :: Int
, _str :: String
, _char :: Char
}
makeLenses ''Example
int :: Lens' Example Int
str :: Lens' Example String
char :: Lens' Example Char
However, we can actually go a step further and completely refactor our Example type like this:
data Example = Example
{ _example2 :: Example
, _char :: Char
}
data Example2 = Example2
{ _int2 :: Int
, _str2 :: String
}
makeLenses ''Example
char :: Lens' Example Char
example2 :: Lens' Example Example2
makeLenses ''Example2
int2 :: Lens' Example2 Int
str2 :: Lens' Example2 String
Do we have to break our old code? No! All we have to do is add the following two lenses to support backwards compatibility:
int :: Lens' Example Int
int = example2 . int2
str :: Lens' Example Char
str = example2 . str2
Now all the old code still works without any changes, despite the intrusive refactoring of our Example type.
In fact, this works for more than just records. You can do the exact same thing for sum types, too (a.k.a. algebraic data types or enums). For example, suppose we have this type:
data Example3 = A String | B Int
makeTraversals ''Example3
-- This creates these `Traversals'`:
_A :: Traversal' Example3 String
_B :: Traversal' Example3 Int
Many of the things that we did with sum types can similarly be re-expressed in terms of Traversal's. There's a notable exception of pattern matching: it's actually possible to implement pattern matching with totality checking with Traversals, but it's currently verbose.
However, the same point holds: if you express all your sum type operations in terms of Traversal's, then you can greatly refactor your sum type and just update the appropriate Traversal's to preserve backwards compatibility.
Finally: note that the true analog of sum type constructors are Prisms (which let you build values using the constructors in addition to pattern matching). Those are not supported by the lens-family family of libraries, but they are provided by lens and you can implement them yourself using just a profunctors dependency if you want.
Also, if you're wondering what the lens analog of a newtype is, it's an Iso', and that also minimally requires a profunctors dependency.
Also, everything I've said works for reference multiple fields of recursive types (using Folds). Literally anything you can imagine wanting to reference in a data type in a backwards-compatible way is encompassed by the lens library.