Despite the title I'm not going to ask about a mere translation between OO world and Haskell, but I can't figure out a better title. This discussion is similar, but not equal, to this one.
I've started a toy project just to expand my limited knowledge of Haskell while reading "Learn You a Haskell for a Great Good", and I've decided to implement a very basic "Elemental Type System", which is a subset of a typical battle system in games like Final Fantasy et simila.
I'm skipping most of the details, but this is in a nutshell my problem:
I want to model a spell, a magic you can cast on the player or on a monster. In the OO world you usually go for a "Castable" interface with a method "onCast(Player)", a "Spell" class so you can define thing like this
Spell myNewSpell = Spell("Fire", 100, 20);
myNewSpell.onCast(Player p); //models the behaviour for the Fire spell
In Haskell I thought this in terms of Types and Classes (I know that Classes in Haskell are a different concept!). I've encountered some difficulties, because my first attempt was to create this:
--A type synonim, a tuple (HP,MP)
type CastResult = (Integer,Integer)
--A castable spell can either deal damage (or restore) or
--inflict a status
class Castable s where
onCast :: s -> Either (Maybe Status) CastResult
data Spell = Spell{spellName :: String,
spellCost :: Integer,
spellHpDmg :: Integer,
spellMpDmg :: Integer,
spellElem :: Maybe Element} deriving (Eq,Show,Read)
Now suppose I create some spell using the Record Syntax
bio = Spell{spellName = "Bio", ...etc..}
I would like be able to do something like this
instance Castable bio where
onCast bio = Left (Just Poison)
There are many problems here:
I can't do "Castable bio" since bio must be a concrete type, not a value of the Type (It should be Castable Spell)
bio isn't in scope, inside the instance block is seen just as a value to pattern match against
Overall, I feel this choice of design is pretty poor, but I'm still learning and I don't grasp such advanced topics like Functors, just to name one.
In a nutshell, which is the idiomatic way to dealing with situation like this? I mean situation which requires "one definition, multiple implementation for multiple instances", just to use the OO terminology.
Type classes are useful when you're dealing with different types. In this case, however, it seems to me like you're dealing with separate instances. In such a case, it's probably simplest to have the cast function be just another record field.
data Spell = Spell{spellName :: String,
...
onCast :: Either (Maybe Status) CastResult }
deriving (Eq,Show,Read)
bio = Spell { spellName = "Bio", onCast = Left (Just Poison), ... }
Or you could do something that models your requirements more explicitly, using domain-specific types rather than generic ones like Either.
type ManaPoints = Integer
type HitPoints = Integer
data Spell = Spell { spellName :: String,
spellCost :: ManaPoints,
spellElem :: Maybe Element,
spellEffect :: Effect }
data Effect = Damage HitPoints ManaPoints
| Inflict Status
cast :: Spell -> Player -> Player
cast spell player =
case spellEffect spell of
Damage hp mana = ...
Inflict status = ...
bio = Spell { spellName = "Bio", spellEffect = Inflict Poison, ... }
fire = Spell { spellName = "Fire", spellEffect = Damage 100 0, ... }
data Spell = Spell{ spellName :: String
, spellCost :: Integer
, spellHpDmg :: Integer
, spellMpDmg :: Integer
, spellElem :: Maybe Element
, spellStatus :: Maybe Status
}
deriving (Eq,Show,Read)
class Castable s where
onCast :: s -> (CastResult, Maybe Status)
instance Castable Spell where
onCast s = ((spellHpDmg s, spellMgDmg s), spellStatus s)
This would probably do the trick here, not sure whether the class is useful in this case though. Is something else than a Spell castable? Something like a Skill, or an Item?
If I understand you correctly, I think you should make onCast an additional record field of Spell, then you can write:
bio = Spell{spellName = "Bio", ...etc.., onCast = Left (Just Poison)}
You won't be able to do deriving (Eq,Show,Read) anymore though, as Spell now contains a function type. You'll have to write those instances manually. Edit: actually onCast isn't a function type, so ignore this.
Related
I'm making a toy forum to gain familiarity with Haskell and Servant.
My API looks something like this:
type UserAPI = "messages" :> ReqBody '[JSON] Msg :> Header "X-Real-IP" String :> Post '[JSON] APIMessage
:<|> "messages" :> ReqBody '[JSON] Int :> Get '[JSON] [Msg']
My types look something like this:
data Msg = Msg
{ thread :: Int
, dname :: String
, contents :: String
} deriving (Eq, Show, Generic)
data Msg' = Msg'
{ thread' :: Int
, stamp' :: UTCTime
, dname' :: String
, contents' :: String
, ip' :: String
} deriving (Eq, Show, Generic)
and they derive ToJSON / FromJSON / FromRow instances, which is very convenient.
Msg represents the data the API expects when receiving messages and Msg' the data it sends when queried for messages, which has two additional fields that are added by the server, but this doesn't feel right and there has to be a cleaner way to achieve this.
Any insight on an idiomatic way to do deal with this sort of problem appreciated.
I will consider here that you question is more a conceptual one ("What can I do when I have two data types that share some structure ?") than a simple "How do I model inheritance in Haskell ?" that is already replied here.
To answer your question, you will need to consider more than just the structure of your data. For example, if I provide you A and B and if I state that
data A = A Int String
data B = B Int
I doubt that you will automatically make the assumption that a A is a B with an extra String. You will probably try to figure the exact relation between these two data structure. And this is the good thing to do.
If each instance of A can actually be seen as an instance of B then it can be relevant to provide way to represent it in your code. Then you could use a plain Haskell way with a
data A = A { super :: B, theString :: String }
data B = B { id :: Int }
Obviously, this will not be easy to work with these datatype without creating some other functions. For example a fromB function could be relevant
fromB :: B -> String -> A
toB :: A -> B
And you can also use typeclass to access id
class HasId a where
getId :: a -> Int
instance HasId A where
getId = id . super
This is where some help form Lens can be useful. And the answer to this question How do I model inheritance in Haskell? is a good start. Lens package provides Object Oriented syntactic sugar to handle inheritance relationship.
However you can also find that a A is not exactly a B but they both share the same ancestor. And you could prefer to create something like
data A = A { share :: C, theString :: String }
data B = B { share :: C }
data C = C Int
This is the case when you do not want to use a A as a B, but it exists some function that can be used by both. The implementation will be near the previous cases, so I do not explain it.
Finally you could find that there does not really exists relation that can be useful (and, therefore, no function that will really exists that is shared between A and B). Then you would prefer to keep your code.
In your specific case, I think that there is not a direct "is a" relation between Msg and Msg' since one is for the receiving and the other is for the sending. But they could share a common ancestor since both are messages. So they will probably have some constructors in common and accessors (in term of OO programming).
Try to never forget that structure is always bind to some functions. And what category theory teaches us is that you cannot only look at the structures only but you have to consider their functions also to see the relation between each other.
this post is the following of this one.
I'm realizing a simple battle system as toy project, the typical system you can find in games like Final Fantasy et simila. I've solved the notorious "Namespace Pollution" problem with a class type + custom instances. For example:
type HitPoints = Integer
type ManaPoints = Integer
data Status = Sleep | Poison | .. --Omitted
data Element = Fire | ... --Omitted
class Targetable a where
name :: a -> String
level :: a -> Int
hp :: a -> HitPoints
mp :: a -> ManaPoints
status :: a -> Maybe [Status]
data Monster = Monster{monsterName :: String,
monsterLevel :: Int,
monsterHp :: HitPoints,
monsterMp :: ManaPoints,
monsterElemType :: Maybe Element,
monsterStatus :: Maybe [Status]} deriving (Eq, Read)
instance Targetable Monster where
name = monsterName
level = monsterLevel
hp = monsterHp
mp = monsterMp
status = monsterStatus
data Player = Player{playerName :: String,
playerLevel :: Int,
playerHp :: HitPoints,
playerMp :: ManaPoints,
playerStatus :: Maybe [Status]} deriving (Show, Read)
instance Targetable Player where
name = playerName
level = playerLevel
hp = playerHp
mp = playerMp
status = playerStatus
Now the problem: I have a spell type, and a spell can deal damage or inflict a status (like Poison, Sleep, Confusion, etc):
--Essentially the result of a spell cast
data SpellEffect = Damage HitPoints ManaPoints
| Inflict [Status] deriving (Show)
--Essentially a magic
data Spell = Spell{spellName :: String,
spellCost :: Integer,
spellElem :: Maybe Element,
spellEffect :: SpellEffect} deriving (Show)
--For example
fire = Spell "Fire" 20 (Just Fire) (Damage 100 0)
frogSong = Spell "Frog Song" 30 Nothing (Inflict [Frog, Sleep])
As suggested in the linked topic, I've created a generic "cast" function like this:
--cast function
cast :: (Targetable t) => Spell -> t -> t
cast s t =
case spellEffect s of
Damage hp mana -> t
Inflict statList -> t
As you can see the return type is t, here showed just for consistency. I want be able to return a new targetable (i.e. a Monster or a Player) with some field value altered (for example a new Monster with less hp, or with a new status). The problem is that i can't just to the following:
--cast function
cast :: (Targetable t) => Spell -> t -> t
cast s t =
case spellEffect s of
Damage hp' mana' -> t {hp = hp', mana = mana'}
Inflict statList -> t {status = statList}
because hp, mana and status "are not valid record selector". The problem is that I don't know a priori if t will be a monster or a player, and I don't want to specify "monsterHp" or "playerHp", I want to write a pretty generic function.
I know that Haskell Records are clumsy and not much extensibile...
Any idea?
Bye and happy coding,
Alfredo
Personally, I think hammar is on the right track with pointing out the similarities between Player and Monster. I agree you don't want to make them the same, but consider this: Take the type class you have here...
class Targetable a where
name :: a -> String
level :: a -> Int
hp :: a -> HitPoints
mp :: a -> ManaPoints
status :: a -> Maybe [Status]
...and replace it with a data type:
data Targetable = Targetable { name :: String
, level :: Int
, hp :: HitPoints
, mp :: ManaPoints
, status :: Maybe [Status]
} deriving (Eq, Read, Show)
Then factor out the common fields from Player and Monster:
data Monster = Monster { monsterTarget :: Targetable
, monsterElemType :: Maybe Element,
} deriving (Eq, Read, Show)
data Player = Player { playerTarget :: Targetable } deriving (Eq, Read, Show)
Depending on what you do with these, it might make more sense to turn it inside-out instead:
data Targetable a = Targetable { target :: a
, name :: String
-- &c...
}
...and then have Targetable Player and Targetable Monster. The advantage here is that any functions that work with either can take things of type Targetable a--just like functions that would have taken any instance of the Targetable class.
Not only is this approach nearly identical to what you have already, it's also a lot less code, and keeps the types simpler (by not having class constraints everywhere). In fact, the Targetable type above is roughly what GHC creates behind the scenes for the type class.
The biggest downside to this approach is that it makes accessing fields clumsier--either way, some things end up being two layers deep, and extending this approach to more complicated types can nest them deeper still. A lot of what makes this awkward is the fact that field accessors aren't "first class" in the language--you can't pass them around like functions, abstract over them, or anything like that. The most popular solution is to use "lenses", which another answer mentioned already. I've typically used the fclabels package for this, so that's my recommendation.
The factored-out types I suggest, combined with strategic use of lenses, should give you something that's simpler to use than the type class approach, and doesn't pollute the namespace the way having lots of record types does.
I can suggest three possible solutions.
1) Your types are very OO-like, but Haskell can also express "sum" types with parameters:
data Unit = UMon Monster | UPlay Player
cast :: Spell -> Unit -> Unit
cast s t =
case spellEffect s of
Damage hp' mana' -> case t of
UMon m -> UMon (m { monsterHp = monsterHp m - hp', monsterMana = undefined})
UPluy p -> UPlay (p { playerHp = playerHp p - hp'})
Inflict statList -> undefined
Thing that are similar in OO-design often become "sum" types with parameters in Haskell.
2) You can do what Carston suggests and add all your methods to type classes.
3) You can change your read-only methods in Targetable to be "lenses" that expose both getting and setting. See the stack overflow discussion. If your type class returned lenses then it would make your spell damage possible to apply.
Why don't you just include functions like
InflicteDamage :: a -> Int -> a
AddStatus :: a -> Status -> a
into your type-class?
I have two records that both have a field I want to extract for display. How do I arrange things so they can be manipulated with the same functions? Since they have different fields (in this case firstName and buildingName) that are their name fields, they each need some "adapter" code to map firstName to name. Here is what I have so far:
class Nameable a where
name :: a -> String
data Human = Human {
firstName :: String
}
data Building = Building {
buildingName :: String
}
instance Nameable Human where
name x = firstName x
instance Nameable Building where
-- I think the x is redundant here, i.e the following should work:
-- name = buildingName
name x = buildingName x
main :: IO ()
main = do
putStr $ show (map name items)
where
items :: (Nameable a) => [a]
items = [ Human{firstName = "Don"}
-- Ideally I want the next line in the array too, but that gives an
-- obvious type error at the moment.
--, Building{buildingName = "Empire State"}
]
This does not compile:
TypeTest.hs:23:14:
Couldn't match expected type `a' against inferred type `Human'
`a' is a rigid type variable bound by
the type signature for `items' at TypeTest.hs:22:23
In the expression: Human {firstName = "Don"}
In the expression: [Human {firstName = "Don"}]
In the definition of `items': items = [Human {firstName = "Don"}]
I would have expected the instance Nameable Human section would make this work. Can someone explain what I am doing wrong, and for bonus points what "concept" I am trying to get working, since I'm having trouble knowing what to search for.
This question feels similar, but I couldn't figure out the connection with my problem.
Consider the type of items:
items :: (Nameable a) => [a]
It's saying that for any Nameable type, items will give me a list of that type. It does not say that items is a list that may contain different Nameable types, as you might think. You want something like items :: [exists a. Nameable a => a], except that you'll need to introduce a wrapper type and use forall instead. (See: Existential type)
{-# LANGUAGE ExistentialQuantification #-}
data SomeNameable = forall a. Nameable a => SomeNameable a
[...]
items :: [SomeNameable]
items = [ SomeNameable $ Human {firstName = "Don"},
SomeNameable $ Building {buildingName = "Empire State"} ]
The quantifier in the data constructor of SomeNameable basically allows it to forget everything about exactly which a is used, except that it is Nameable. Therefore, you will only be allowed to use functions from the Nameable class on the elements.
To make this nicer to use, you can make an instance for the wrapper:
instance Nameable (SomeNameable a) where
name (SomeNameable x) = name x
Now you can use it like this:
Main> map name items
["Don", "Empire State"]
Everybody is reaching for either existential quantification or algebraic data types. But these are both overkill (well depending on your needs, ADTs might not be).
The first thing to note is that Haskell has no downcasting. That is, if you use the following existential:
data SomeNameable = forall a. Nameable a => SomeNameable a
then when you create an object
foo :: SomeNameable
foo = SomeNameable $ Human { firstName = "John" }
the information about which concrete type the object was made with (here Human) is forever lost. The only things we know are: it is some type a, and there is a Nameable a instance.
What is it possible to do with such a pair? Well, you can get the name of the a you have, and... that's it. That's all there is to it. In fact, there is an isomorphism. I will make a new data type so you can see how this isomorphism arises in cases when all your concrete objects have more structure than the class.
data ProtoNameable = ProtoNameable {
-- one field for each typeclass method
protoName :: String
}
instance Nameable ProtoNameable where
name = protoName
toProto :: SomeNameable -> ProtoNameable
toProto (SomeNameable x) = ProtoNameable { protoName = name x }
fromProto :: ProtoNameable -> SomeNameable
fromProto = SomeNameable
As we can see, this fancy existential type SomeNameable has the same structure and information as ProtoNameable, which is isomorphic to String, so when you are using this lofty concept SomeNameable, you're really just saying String in a convoluted way. So why not just say String?
Your items definition has exactly the same information as this definition:
items = [ "Don", "Empire State" ]
I should add a few notes about this "protoization": it is only as straightforward as this when the typeclass you are existentially quantifying over has a certain structure: namely when it looks like an OO class.
class Foo a where
method1 :: ... -> a -> ...
method2 :: ... -> a -> ...
...
That is, each method only uses a once as an argument. If you have something like Num
class Num a where
(+) :: a -> a -> a
...
which uses a in multiple argument positions, or as a result, then eliminating the existential is not as easy, but still possible. However my recommendation to do this changes from a frustration to a subtle context-dependent choice, because of the complexity and distant relationship of the two representations. However, every time I have seen existentials used in practice it is with the Foo kind of tyepclass, where it only adds needless complexity, so I quite emphatically consider it an antipattern. In most of these cases I recommend eliminating the entire class from your codebase and exclusively using the protoized type (after you give it a good name).
Also, if you do need to downcast, then existentials aren't your man. You can either use an algebraic data type, as others people have answered, or you can use Data.Dynamic (which is basically an existential over Typeable. But don't do that; a Haskell programmer resorting to Dynamic is ungentlemanlike. An ADT is the way to go, where you characterize all the possible types it could be in one place (which is necessary so that the functions that do the "downcasting" know that they handle all possible cases).
I like #hammar's answer, and you should also check out this article which provides another example.
But, you might want to think differently about your types. The boxing of Nameable into the SomeNameable data type usually makes me start thinking about whether a union type for the specific case is meaningful.
data Entity = H Human | B Building
instance Nameable Entity where ...
items = [H (Human "Don"), B (Building "Town Hall")]
I'm not sure why you want to use the same function for
getting the name of a Human and the name of a Building.
If their names are used in fundamentally different ways,
except maybe for simple things like printing them,
then you probably want two
different functions for that. The type system
will automatically guide you to choose the right function
to use in each situation.
But if having a name is something significant about the
whole purpose of your program, and a Human and a Building
are really pretty much the same thing in that respect as far as your program
is concerned, then you would define their type together:
data NameableThing =
Human { name :: String } |
Building { name :: String }
That gives you a polymorphic function name that works for
whatever particular flavor of NameableThing you happen to have,
without needing to get into type classes.
Usually you would use a type class for a different kind of situation:
if you have some kind of non-trivial operation that has the same purpose
but a different implementation for several different types.
Even then, it's often better to use some other approach instead, like
passing a function as a parameter (a "higher order function", or "HOF").
Haskell type classes are a beautiful and powerful tool, but they are totally
different than what is called a "class" in object-oriented languages,
and they are used far less often.
And I certainly don't recommend complicating your program by using an advanced
extension to Haskell like Existential Qualification just to fit into
an object-oriented design pattern.
You can try to use Existentially Quanitified types and do it like this:
data T = forall a. Nameable a => MkT a
items = [MkT (Human "bla"), MkT (Building "bla")]
I've just had a look at the code that this question is abstracting from. For this, I would recommend merging the Task and RecurringTaskDefinition types:
data Task
= Once
{ name :: String
, scheduled :: Maybe Day
, category :: TaskCategory
}
| Recurring
{ name :: String
, nextOccurrence :: Day
, frequency :: RecurFrequency
}
type ProgramData = [Task] -- don't even need a new data type for this any more
Then, the name function works just fine on either type, and the functions you were complaining about like deleteTask and deleteRecurring don't even need to exist -- you can just use the standard delete function as usual.
how can i group getX and putX in a class instance ?
the code below is an answer for this post Class set method in Haskell using State-Monad
import Control.Monad.State
data Point = Point { x :: Int, y :: Int } deriving Show
getX :: State Point Int
getX = get >>= return . x
putX :: Int -> State Point ()
putX newVal = do
pt - get
put (pt { x = newVal })
increaseX :: State Point ()
increaseX = do
x - getX
putX (x + 1)
Later I hope I will implement setters and getters for a hierarchy of 2 classes, but for now i just wanna do something like this:
class A a where
putX :: Int -> State Point ()
instance A (State Point) where
putX newVal = do
pt - get
put (pt { x = newVal })
You seem to be conflating multiple concepts here, to the point that I'm not sure exactly what you're aiming to accomplish. A few thoughts on what you might be after:
Field access, i.e., a way to inspect or replace a piece of a larger data structure. This doesn't really lend itself to a type class, because for many combinations of "inner field" and "data structure" there will be more than one accessor possible. Abstractions along these lines are often called "lenses".
Stateful references in some generic fashion. For the most part in a State monad this amounts to combining something like the aforementioned lenses with the standard get and put. In this case you could have a type class for the combination of a particular monad and the accessor data type, but it wouldn't really do that much.
Overloading access to a particular field, so that functions can work on any data type that contains an "x" field. In this case I assume you'd also want "y", as some sort of type class for 2D points. This is entirely separate from the above issues, however.
Perhaps you could clarify your goal?
Record syntax seems extremely convenient compared to having to write your own accessor functions. I've never seen anyone give any guidelines as to when it's best to use record syntax over normal data declaration syntax, so I'll just ask here.
You should use record syntax in two situations:
The type has many fields
The type declaration gives no clue about its intended layout
For instance a Point type can be simply declared as:
data Point = Point Int Int deriving (Show)
It is obvious that the first Int denotes the x coordinate and the second stands for y. But the case with the following type declaration is different (taken from Learn You a Haskell for Great Good):
data Person = Person String String Int Float String String deriving (Show)
The intended type layout is: first name, last name, age, height, phone number, and favorite ice-cream flavor. But this is not evident in the above declaration. Record syntax comes handy here:
data Person = Person { firstName :: String
, lastName :: String
, age :: Int
, height :: Float
, phoneNumber :: String
, flavor :: String
} deriving (Show)
The record syntax made the code more readable, and saved a great deal of typing by automatically defining all the accessor functions for us!
In addition to complex multi-fielded data, newtypes are often defined with record syntax. In either of these cases, there aren't really any downsides to using record syntax, but in the case of sum types, record accessors usually don't make sense. For example:
data Either a b = Left { getLeft :: a } | Right { getRight :: b }
is valid, but the accessor functions are partial – it is an error to write getLeft (Right "banana"). For that reason, such accessors are generally speaking discouraged; something like getLeft :: Either a b -> Maybe a would be more common, and that would have to be defined manually. However, note that accessors can share names:
data Item = Food { description :: String, tastiness :: Integer }
| Wand { description :: String, magic :: Integer }
Now description is total, although tastiness and magic both still aren't.