I cannot wrap my head around how to define a type in Haskell that represents the recursive nature of an HTTP Multipart MIME POST.
In English, a Post is either a list of Headers along with Content of some type, or it's a list of Headers with Content of another Post. But Content can also be a list of Posts.
So I've defined Header thus:
data Header = Header { hName :: String
, hValue :: String
, hAddl :: [(String,String)] } deriving (Eq, Show)
I guess Content should be something like:
data Content a = Content a | [Post] deriving (Eq, Show)
Obviously, that fails: parse error in constructor in data/newtype declaration: [Post]
I've defined Post as:
data Post = Post { pHeaders :: [Header]
, pContent :: [Content] } deriving (Eq, Show)
I'm using Haskell to get a different perspective on my current task, the latest question thereon being here. Just using String for Content, I can parse simple POSTs using Parsec. But the goal is to parse complex Posts.
The link above, and the links found at that question, give the context for my current task. I'm a Haskell hobbyist, so please feel free to offer alternatives to the code I've posted here--I'm not married to it, and I'd love to learn. Ultimately, I'll use F#, unless I am unable to deliver, in which case I'll be forced to use C# and an imperative style. I welcome any wisdom or direction that supports a functional solution!
Your datatypes make sense, your syntax is just wrong:
data Content a = Content a | Posts [Post a] deriving (Eq, Show)
You can name the Posts constructor whatever you like. However, you cannot have something like pContent :: [Content] - since content has a type variable, it must be applied to a type:
data Post a = Post { pHeaders :: [Header]
, pContent :: [Content a] } deriving (Eq, Show)
I would say that your approach is idiomatic Haskell.
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.
I'm writing a parser in Haskell (mostly just to learn). I have a working tokenizer and parser and I want to add line numbers when giving an error message. I have this type:
data Token = Lambda
| Dot
| LParen
| RParen
| Ident String
Back in OO land, I would just create a Metadata object that holds the token's position in the source code. So I could try this:
data Metadata = Pos String Int Int
Then, I could change Token to
data Token = Lambda Metadata
| Dot Metadata
| LParen Metadata
| RParen Metadata
| Ident String Metadata
However, my parser is written using pattern matching on the tokens. So now, all my pattern matching is broken because I need to also account for the Metadata. So that doesn't seem ideal. 99% of the time, I don't care about the Metadata.
So what's the "right" way to do what I want to do?
There’s a wide array of approaches to the design of syntax representations in Haskell, but I can offer some recommendations and reasoning.
It’s advisable to keep metadata annotations out of the Token type, so that it sticks to a single responsibility. If a Token represents just a token, its derived instances for Eq and so on will work as expected without needing to worry about when to ignore the annotation.
Thankfully, the alternatives are simple in this case. One option is to move the annotation info to a separate wrapper type.
-- An #'Anno' a# is a value of type #a# annotated with some 'Metadata'.
data Anno a = Anno { annotation :: Metadata, item :: a }
deriving
( Eq
, Ord
, Show
-- …
)
Now the tokeniser can return a sequence of annotated tokens, i.e. [Annotated Token]. You still need to update the use sites, but the changes are now much simpler. And you can ignore annotations in various ways:
-- Positional matching
f1 (Anno _meta (Ident name)) = …
-- Record matching
f2 Anno { item = Ident name } = …
-- With ‘NamedFieldPuns’
f3 Anno { item } = …
-- 'U'nannotated value; with ‘PatternSynonyms’
pattern U :: a -> Anno a
pattern U x <- Anno _meta x
f4 (U LParen) = …
You can deannotate a sequence of tokens with fmap item to reuse existing code that doesn’t care about location info. And since Anno is a type of kind Type -> Type, GHC can also derive Foldable, Functor, and Traversable for it, making it easy to operate on the annotated item with e.g. fmap and traverse.
This is the preferable approach for Token, but for a parsed AST containing annotations, you may want to make the annotation type a parameter of the AST type, for example:
data Expr a = Add a (Expr a) (Expr a) | Literal a Int
deriving (Eq, Foldable, Functor, Ord, Show, Traversable)
Then you can use Expr Metadata for an annotated term, or Expr () for an unannotated one. To compare terms for equality, such as in unit tests, you can use the Functor instance to strip out the annotations, e.g. void expr1 == void expr2, where void is equivalent to fmap (\ _meta -> ()) here.
In a larger codebase, if there’s a lot of code depending on a data type and you really want to avoid updating it all at once, you can wrap the old type in a module that exports a pattern synonym for each of the old constructors. This lets you gradually update the old code before deleting the adapter module.
Culturally, it’s typical in a self-contained Haskell codebase to simply make breaking changes, and let the compiler tell you everywhere that needs to be updated, since it’s so easy to do extensive refactoring with high assurance that it’s correct. We’re more concerned with backward compatibility when it comes to published library code, since that actually affects other people.
Noob here again. Getting my feet wet in making my own Haskell 'programs'. Stumbled across this.
Made my own type:
data Action = Action
{ idN :: IdN
, description :: Desc
, duedate :: Due
, donedate :: Done
} deriving (Ord, Show, Read, Eq)
Imported Data.Text.IO. Want to write concrete info in Action to file using
TIO.appendFile "./Dbase.txt" typedAction
where typedAction is the concrete representation of the Action type. Now Action is not of type Text.
So how would I go about this?
I'm currently building a a Twitter CLI client in Haskell, and I have a data type that represents a DM and one that represents a tweet. However, I get a multiple declaration error because I have to use the same name for both:
data Users = Users { screen_name :: String } deriving(Show, Generic)
data Tweet = Tweet { text :: !Text,
retweeted :: Bool,
user :: Users
} deriving (Show, Generic)
data DM = DM { text :: !Text,
sender_screen_name :: String
} deriving (Show, Generic)
Does someone know a solution for this particular problem?
As defined here, the named members are just functions that are used to call the values in your data structure.
So, if you really want to use them, you can do so by using the language extension. You can do that by declaring this in your file:
{-# LANGUAGE DuplicateRecordFields #-}
I have two data types, which are used for hastache templates. It makes sense in my code to have two different types, both with a field named "name". This, of course, causes a conflict. It seems that there's a mechanism to disambiguate any calls to "name", but the actual definition causes problems. Is there any workaround, say letting the record field name be qualified?
data DeviceArray = DeviceArray
{ name :: String,
bytes :: Int }
deriving (Eq, Show, Data, Typeable)
data TemplateParams = TemplateParams
{ arrays :: [DeviceArray],
input :: DeviceArray }
deriving (Eq, Show, Data, Typeable)
data MakefileParams = MakefileParams
{ name :: String }
deriving (Eq, Show, Data, Typeable)
i.e. if the fields are now used in code, they will be "DeviceArray.name" and "MakefileParams.name"?
As already noted, this isn't directly possible, but I'd like to say a couple things about proposed solutions:
If the two fields are clearly distinct, you'll want to always know which you're using anyway. By "clearly distinct" here I mean that there would never be a circumstance where it would make sense to do the same thing with either field. Given this, excess disambiguity isn't really unwelcome, so you'd want either qualified imports as the standard approach, or the field disambiguation extension if that's more to your taste. Or, as a very simplistic (and slightly ugly) option, just manually prefix the fields, e.g. deviceArrayName instead of just name.
If the two fields are in some sense the same thing, it makes sense to be able to treat them in a homogeneous way; ideally you could write a function polymorphic in choice of name field. In this case, one option is using a type class for "named things", with functions that let you access the name field on any appropriate type. A major downside here, besides a proliferation of trivial type constraints and possible headaches from the Dreaded Monomorphism Restriction, is that you also lose the ability to use the record syntax, which begins to defeat the whole point.
The other major option for similar fields, which I didn't see suggested yet, is to extract the name field out into a single parameterized type, e.g. data Named a = Named { name :: String, item :: a }. GHC itself uses this approach for source locations in syntax trees, and while it doesn't use record syntax the idea is the same. The downside here is that if you have a Named DeviceArray, accessing the bytes field now requires going through two layers of records. If you want to update the bytes field with a function, you're stuck with something like this:
addBytes b na = na { item = (item na) { bytes = b + bytes (item na) } }
Ugh. There are ways to mitigate the issue a bit, but they're still not idea, to my mind. Cases like this are why I don't like record syntax in general. So, as a final option, some Template Haskell magic and the fclabels package:
{-# LANGUAGE TemplateHaskell #-}
import Control.Category
import Data.Record.Label
data Named a = Named
{ _name :: String,
_namedItem :: a }
deriving (Eq, Show, Data, Typeable)
data DeviceArray = DeviceArray { _bytes :: Int }
deriving (Eq, Show, Data, Typeable)
data MakefileParams = MakefileParams { _makefileParams :: [MakeParam] }
deriving (Eq, Show, Data, Typeable)
data MakeParam = MakeParam { paramText :: String }
deriving (Eq, Show, Data, Typeable)
$(mkLabels [''Named, ''DeviceArray, ''MakefileParams, ''MakeParam])
Don't mind the MakeParam business, I just needed a field on there to do something with. Anyway, now you can modify fields like this:
addBytes b = modL (namedItem >>> bytes) (b +)
nubParams = modL (namedItem >>> makefileParams) nub
You could also name bytes something like bytesInternal and then export an accessor bytes = namedItem >>> bytesInternal if you like.
Record field names are in the same scope as the data type, so you cannot do this directly.
The common ways to work around this is to either add prefixes to the field names, e.g. daName, mpName, or put them in separate modules which you then import qualified.
What you can do is to put each data type in its own module, then you can used qualified imports to disambiguate. It's a little clunky, but it works.
There are several GHC extensions which may help. The linked one is applicable in your case.
Or, you could refactor your code and use typeclasses for the common fields in records. Or, you should manually prefix each record selector with a prefix.
If you want to use the name in both, you can use a Class that define the name funcion. E.g:
Class Named a where
name :: a -> String
data DeviceArray = DeviceArray
{ deviceArrayName :: String,
bytes :: Int }
deriving (Eq, Show, Data, Typeable)
instance Named DeviceArray where
name = deviceArrayName
data MakefileParams = MakefileParams
{ makefileParamsName :: String }
deriving (Eq, Show, Data, Typeable)
instance Named MakefileParams where
name = makefileParamsName
And then you can use name on both classes.