Self-describing consumers/parsers; forms, cmd-line args, JSON, etc.

Version on this page:0.9.4
LTS Haskell 11.22:0.9.4
Stackage Nightly 2022-02-07:0.9.5
Latest on Hackage:0.9.5

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BSD-3-Clause licensed by Chris Done
Maintained by [email protected]
This version can be pinned in stack with:descriptive-0.9.4@sha256:7ff9622835a729da6b19712cbe95f02a3343911da81cbf73e0327a68ff66da67,1561


Self-describing consumers/parsers


There are a variety of Haskell libraries which are implementable through a common interface: self-describing parsers:

  • A formlet is a self-describing parser.
  • A regular old text parser can be self-describing.
  • A command-line options parser is a self-describing parser.
  • A MUD command set is a self-describing parser.
  • A JSON API can be a self-describing parser.

Consumption is done in this data type:

data Consumer s d m a

Making descriptive consumers

To make a consumer, this combinator is used:

consumer :: (StateT s m (Description d))
         -- ^ Produce description based on the state.
         -> (StateT s m (Result (Description d) a))
         -- ^ Parse the state and maybe transform it if desired.
         -> Consumer s d m a

The first argument generates a description based on some state. The state is determined by whatever use-case you have. The second argument parses from the state, which could be a stream of bytes, a list of strings, a Map, a Vector, etc. You may or may not decide to modify the state during generation of the description and during parsing.

Running descriptive consumers

To use a consumer or describe what it does, these are used:

consume :: Consumer s d Identity a -- ^ The consumer to run.
        -> s -- ^ Initial state.
        -> Result (Description d) a

describe :: Consumer s d Identity a -- ^ The consumer to run.
         -> s -- ^ Initial state. Can be \"empty\" if you don't use it for
              -- generating descriptions.
         -> Description d -- ^ A description and resultant state.

Alternatively the parser/printer can be run in a monad of your choice:

runConsumer :: Monad m
            => Consumer s d m a -- ^ The consumer to run.
            -> StateT s m (Result (Description d) a)

runDescription :: Monad m
               => Consumer s d m a -- ^ The consumer to run.
               -> StateT s m (Description d) -- ^ A description and resultant state.


A description is like this:

data Description a
  = Unit !a
  | Bounded !Integer !Bound !(Description a)
  | And !(Description a) !(Description a)
  | Or !(Description a) !(Description a)
  | Sequence ![Description a]
  | Wrap a !(Description a)
  | None

You configure the a for your use-case, but the rest is generatable by the library. Afterwards, you can make your own pretty printing function, which may be to generate an HTML form, to generate a commandline --help screen, a man page, API docs for your JSON parser, a text parsing grammar, etc. For example:

describeParser :: Description Text -> Text
describeForm :: Description (Html ()) -> Html ()
describeArgs :: Description CmdArgs -> Text


One can wrap up a consumer to alter either the description or the parser or both, this can be used for wrapping labels, or adding validation, things of that nature:

wrap :: (StateT t m (Description d) -> StateT s m (Description d))
     -- ^ Transform the description.
     -> (StateT t m (Description d) -> StateT t m (Result (Description d) a) -> StateT s m (Result (Description d) b))
     -- ^ Transform the parser. Can re-run the parser as many times as desired.
     -> Consumer t d m a
     -> Consumer s d m b

There is also a handy function written in terms of wrap which will validate a consumer.

validate :: Monad m
         => d                           -- ^ Description of what it expects.
         -> (a -> StateT s m (Maybe b)) -- ^ Attempt to parse the value.
         -> Consumer s d m a            -- ^ Consumer to add validation to.
         -> Consumer s d m b            -- ^ A new validating consumer.

See below for some examples of this library.

Parsing characters

See Descriptive.Char.

λ> describe (many (char 'k') <> string "abc") mempty
And (Bounded 0 UnlimitedBound (Unit "k"))
    (Sequence [Unit "a",Unit "b",Unit "c",None])
λ> consume (many (char 'k') <> string "abc") "kkkabc"
(Succeeded "kkkabc")
λ> consume (many (char 'k') <> string "abc") "kkkab"
(Failed (Unit "a character"))
λ> consume (many (char 'k') <> string "abc") "kkkabj"
(Failed (Unit "c"))

Validating forms with named inputs

See Descriptive.Form.

λ> describe ((,) <$> input "username" <*> input "password") mempty
And (Unit (Input "username")) (Unit (Input "password"))

λ> consume ((,) <$>
            input "username" <*>
            input "password")
           (M.fromList [("username","chrisdone"),("password","god")])
Succeeded ("chrisdone","god")

Conditions on two inputs:

login =
  validate "confirmed password (entered the same twice)"
           (\(x,y) ->
              if x == y
                 then Just y
                 else Nothing)
           ((,) <$>
            input "password" <*>
            input "password2") <|>
  input "token"
λ> consume login (M.fromList [("password2","gob"),("password","gob")])
Succeeded "gob"
λ> consume login (M.fromList [("password2","gob"),("password","go")])
Continued (And (Wrap (Constraint "confirmed password (entered the same twice)")
                     (And (Unit (Input "password"))
                          (Unit (Input "password2"))))
               (Unit (Input "token")))
λ> consume login (M.fromList [("password2","gob"),("password","go"),("token","woot")])
Succeeded "woot"

Validating forms with auto-generated input indexes

See Descriptive.Formlet.

λ> describe ((,) <$> indexed <*> indexed)
            (FormletState mempty 0)
And (Unit (Index 0)) (Unit (Index 1))
λ> consume ((,) <$> indexed <*> indexed)
           (FormletState (M.fromList [(0,"chrisdone"),(1,"god")]) 0)
Succeeded ("chrisdone","god")
λ> consume ((,) <$> indexed <*> indexed)
           (FormletState (M.fromList [(0,"chrisdone")]) 0)
Failed (Unit (Index 1))

Parsing command-line options

See Descriptive.Options.

server =
  ((,,,) <$>
   constant "start" "cmd" () <*>
   anyString "SERVER_NAME" <*>
   switch "dev" "Enable dev mode?" <*>
   arg "port" "Port to listen on")
λ> describe server []
And (And (And (Unit (Constant "start"))
               (Unit (AnyString "SERVER_NAME")))
          (Unit (Flag "dev" "Enable dev mode?")))
     (Unit (Arg "port" "Port to listen on"))
λ> consume server ["start","any","--port","1234","--dev"]
Succeeded ((),"any",True,"1234")
λ> consume server ["start","any","--port","1234"]
Succeeded ((),"any",False,"1234")
λ> textDescription (describe server [])
"start SERVER_NAME [--dev] --port <...>"

Self-documenting JSON parser

See Descriptive.JSON.

-- | Submit a URL to reddit.
data Submission =
  Submission {submissionToken :: !Integer
             ,submissionTitle :: !Text
             ,submissionComment :: !Text
             ,submissionSubreddit :: !Integer}
  deriving (Show)

submission :: Monad m => Consumer Value Doc m Submission
submission =
  object "Submission"
           <$> key "token" (integer "Submission token; see the API docs")
           <*> key "title" (string "Submission title")
           <*> key "comment" (string "Submission comment")
           <*> key "subreddit" (integer "The ID of the subreddit"))

sample :: Value
sample =
  toJSON (object
            ["token" .= 123
            ,"title" .= "Some title"
            ,"comment" .= "This is good"
            ,"subreddit" .= 234214])

badsample :: Value
badsample =
  toJSON (object
            ["token" .= 123
            ,"title" .= "Some title"
            ,"comment" .= 123
            ,"subreddit" .= 234214])
λ> describe submission (toJSON ())
Wrap (Object "Submission")
     (And (And (And (Wrap (Key "token")
                          (Unit (Integer "Submission token; see the API docs")))
                    (Wrap (Key "title")
                          (Unit (Text "Submission title"))))
               (Wrap (Key "comment")
                     (Unit (Text "Submission comment"))))
          (Wrap (Key "subreddit")
                (Unit (Integer "The ID of the subreddit"))))

λ> consume submission sample
Succeeded (Submission {submissionToken = 123
                   ,submissionTitle = "Some title"
                   ,submissionComment = "This is good"
                   ,submissionSubreddit = 234214})
λ> consume submission badsample
Failed (Wrap (Object "Submission")
             (Wrap (Key "comment")
                   (Unit (Text "Submission comment"))))

The bad sample yields an informative message that:

  • The error is in the Submission object.
  • The key “comment”.
  • The type of that key should be a String and it should be a Submission comment (or whatever invariants you’d like to mention).

Parsing Attempto Controlled English for MUD commands

TBA. Will use this package.

With ACE you can parse into:

parsed complV "<distrans-verb> a <noun> <prep> a <noun>" ==
Succeeded (ComplVDisV (DistransitiveV "<distrans-verb>")
                  (ComplNP (NPCoordUnmarked (UnmarkedNPCoord anoun Nothing)))
                  (ComplPP (PP (Preposition "<prep>")
                               (NPCoordUnmarked (UnmarkedNPCoord anoun Nothing)))))

Which I can then further parse with descriptive to yield descriptions like:

<verb-phrase> [<noun-phrase> ..]

Or similar. Which would be handy for a MUD so that a user can write:

Put the sword on the table.

Producing questions and consuming the answers in Haskell

TBA. Will be a generalization of this type.

It is a library which I am working on in parallel which will ask the user questions and then validate the answers. Current output is like this:

λ> describe (greaterThan 4 (integerExpr (parse id expr exercise)))
an integer greater than 4
λ> eval (greaterThan 4 (integerExpr (parse id expr exercise))) $(someHaskell "x = 1")
Left expected an expression, but got a declaration
λ> eval (greaterThan 4 (integerExpr (parse id expr exercise))) $(someHaskell "x")
Left expected an integer, but got an expression
λ> eval (greaterThan 4 (integerExpr (parse id expr exercise))) $(someHaskell "3")
Left expected an integer greater than 4
λ> eval (greaterThan 4 (integerExpr (parse id expr exercise))) $(someHaskell "5")
Right 5

This is also couples description with validation, but I will probably rewrite it with this descriptive library.