Errors.hs

-- SPDX-FileCopyrightText: 2021 Oxhead Alpha
-- SPDX-License-Identifier: LicenseRef-MIT-OA

{-# LANGUAGE DeriveLift #-}
{-# LANGUAGE TypeFamilyDependencies #-}

-- We want to make sure 'failUsingArg' is used with sane argument.
{-# OPTIONS_GHC -Wno-redundant-constraints #-}

module Lorentz.Errors
  ( -- * Haskell to 'Value' conversion
    IsError (..)
  , ErrorScope
  , ConstantScope
  , isoErrorToVal
  , isoErrorFromVal
  , simpleFailUsing
  , ErrorHasDoc (..)
  , typeDocMdDescriptionReferToError
  , isInternalErrorClass

  , UnspecifiedError (..)
  , Impossible (..)
  , SomeError (..)

    -- * General instructions
  , failUnexpected

    -- * Custom errors
  , NoErrorArg
  , UnitErrorArg
  , ErrorArg
  , CustomError (..)
  , CustomErrorRep
  , IsCustomErrorArgRep (..)
  , MustHaveErrorArg
  , failCustom
  , failCustom_
  , failCustomNoArg

    -- * Documentation
  , ErrorClass (..)
  , CustomErrorHasDoc (..)
  , DError (..)
  , DThrows (..)

    -- * Internals
  , errorTagToText
  , errorTagToMText
  ) where

import Debug qualified (show)

import Data.Char qualified as C
import Data.List qualified as L
import Fmt (Buildable, build, fmt, pretty, (+|), (|+))
import Language.Haskell.TH.Syntax (Lift)
import Text.Read (readsPrec)
import Text.Show qualified

import Lorentz.Base
import Lorentz.Doc
import Lorentz.Ext
import Lorentz.Instr hiding (cast)
import Lorentz.Value
import Morley.Michelson.Text
import Morley.Michelson.Typed.Convert (untypeValue)
import Morley.Michelson.Typed.Haskell
import Morley.Michelson.Typed.Instr
import Morley.Michelson.Typed.Scope
import Morley.Michelson.Typed.Sing (castM, castSingE)
import Morley.Util.Markdown
import Morley.Util.MismatchError
import Morley.Util.Type
import Morley.Util.TypeLits
import Morley.Util.Typeable

----------------------------------------------------------------------------
-- IsError
----------------------------------------------------------------------------

-- | Since 008 it's prohibited to fail with non-packable values and with the
-- 'Contract t' type values, which is equivalent to our @ConstantScope@ constraint.
-- See https://gitlab.com/tezos/tezos/-/issues/1093#note_496066354 for more information.
type ErrorScope t = ConstantScope t

type KnownError a = ErrorScope (ToT a)

-- | Haskell type representing error.
class (ErrorHasDoc e) => IsError e where

  -- | Converts a Haskell error into @Value@ representation.
  errorToVal :: e -> (forall t. ErrorScope t => Value t -> r) -> r

  -- | Converts a @Value@ into Haskell error.
  errorFromVal :: (SingI t) => Value t -> Either Text e

  -- | Fail with the given Haskell value.
  failUsing :: (IsError e) => e -> s :-> t
  failUsing = simpleFailUsing

-- | Implementation of 'errorToVal' via 'IsoValue'.
isoErrorToVal
  :: (KnownError e, IsoValue e)
  => e -> (forall t. ErrorScope t => Value t -> r) -> r
isoErrorToVal e cont = cont $ toVal e

-- | Implementation of 'errorFromVal' via 'IsoValue'.
isoErrorFromVal
  :: (SingI t, KnownIsoT e, IsoValue e)
  => Value t -> Either Text e
isoErrorFromVal e = fromVal <$> castSingE e

-- | Basic implementation for 'failUsing'.
simpleFailUsing
  :: forall e s t.
     (IsError e)
  => e -> s :-> t
simpleFailUsing err =
  errorToVal err $ \eval ->
    doc (DThrows (Proxy @e)) #
    (FI $ PUSH eval `Seq` FAILWITH)

class Typeable e => ErrorHasDoc (e :: Type) where
  -- | Name of error as it appears in the corresponding section title.
  errorDocName :: Text

  -- | What should happen for this error to be raised.
  errorDocMdCause :: Markdown

  -- | Brief version of 'errorDocMdCause'.
  --
  -- This will appear along with the error when mentioned in entrypoint description.
  -- By default, the first sentence of the full description is used.
  errorDocMdCauseInEntrypoint :: Markdown
  errorDocMdCauseInEntrypoint = pickFirstSentence $ errorDocMdCause @e

  -- | How this error is represented in Haskell.
  errorDocHaskellRep :: Markdown

  -- | Error class.
  errorDocClass :: ErrorClass
  errorDocClass = ErrClassUnknown

  -- | Which definitions documentation for this error mentions.
  errorDocDependencies :: [SomeDocDefinitionItem]

  -- | Constraints which we require in a particular instance.
  -- You are not oblidged to often instantiate this correctly, it is only useful
  -- for some utilities.
  type ErrorRequirements e :: Constraint
  type ErrorRequirements _ = ()

  -- | Captured constraints which we require in a particular instance.
  -- This is a way to encode a bidirectional instance in the nowaday Haskell,
  -- for @class MyConstraint => ErrorHasDoc MyType@ instance it lets deducing
  -- @MyConstraint@ by @ErrorHasDoc MyType@.
  --
  -- You are not oblidged to always instantiate, it is only useful for some
  -- utilities which otherwise would not compile.
  errorDocRequirements :: Dict (ErrorRequirements e)
  default errorDocRequirements :: ErrorRequirements e => Dict (ErrorRequirements e)
  errorDocRequirements = Dict

-- | Helper for managing descriptions.
pickFirstSentence :: Markdown -> Markdown
pickFirstSentence = build . toText . go . fmt
  where
    go :: String -> String
    go = \case
      '.' : c : _ | C.isSpace c -> "."
      c : s -> c : go s
      "" -> ""

----------------------------------------------------------------------------
-- Instances
----------------------------------------------------------------------------

-- | Use this for internal errors only.
--
-- \"Normal\" error scenarios should use the mechanism of custom errors, see below.
instance IsError MText where
  errorToVal = isoErrorToVal
  errorFromVal = isoErrorFromVal

instance ErrorHasDoc MText where
  errorDocName = "InternalError"
  errorDocMdCause =
    "Some internal error occurred."
  errorDocHaskellRep =
    "Textual error message, see " <>
    typeDocMdReference (Proxy @MText) (WithinParens False) <> "."
  errorDocClass = ErrClassContractInternal
  errorDocDependencies = [SomeDocDefinitionItem (DType $ Proxy @MText)]

instance TypeError ('Text "Use representative error messages") => IsError () where
  errorToVal _ _ = error "impossible"
  errorFromVal = error "impossible"

instance TypeError ('Text "Use representative error messages") => ErrorHasDoc () where
  errorDocName = error "impossible"
  errorDocMdCause = error "impossible"
  errorDocHaskellRep = error "impossible"
  errorDocDependencies = error "impossible"

-- | Use this type as replacement for @()@ when you __really__ want to leave
-- error cause unspecified.
data UnspecifiedError = UnspecifiedError
  deriving stock Generic
  deriving anyclass IsoValue

instance IsError UnspecifiedError where
  errorToVal = isoErrorToVal
  errorFromVal = isoErrorFromVal

instance ErrorHasDoc UnspecifiedError where
  errorDocName = "Unspecified error"
  errorDocMdCause = "Some error occurred."
  errorDocHaskellRep = typeDocMdReference (Proxy @()) (WithinParens False) <> "."
  errorDocDependencies = typeDocDependencies (Proxy @())

-- | Use this error when sure that failing at the current position is possible
-- in no curcumstances (including invalid user input or misconfigured storage).
--
-- To use this as error, you have to briefly specify the reason why the error
-- scenario is impossible (experimental feature).
data Impossible (reason :: Symbol) = HasCallStack => Impossible

instance KnownSymbol reason => IsError (Impossible reason) where
  errorToVal Impossible = ($ toVal ())
  errorFromVal = error "Extracting impossible error"

  failUsing err@Impossible =
    justComment codeComment #
    testAssert testDescription "" (push False) #
    simpleFailUsing err
    where
      codeComment = fromString $ "Failure from " <> prettyCallStack callStack
      testDescription =
        "Impossible happened: unexpected " <> symbolValT' @reason <> "\n\
        \At: " <> fromString (prettyCallStack callStack)

instance KnownSymbol reason => ErrorHasDoc (Impossible reason) where
  errorDocName = "Impossible error"
  errorDocMdCause =
    "An impossible error happened.\n\n\
    \If this error occured, contact the contract authors."
  errorDocHaskellRep = typeDocMdReference (Proxy @()) (WithinParens False) <> "."
  errorDocDependencies = typeDocDependencies (Proxy @())

-- | Type wrapper for an @IsError@.
data SomeError = forall e. (IsError e, Eq e) => SomeError e

instance Eq SomeError where
  SomeError e1 == SomeError e2 = eqExt e1 e2

instance Buildable SomeError where
  build (SomeError e) = errorToVal e (build . untypeValue)

instance Show SomeError where
  show = pretty

----------------------------------------------------------------------------
-- General instructions
----------------------------------------------------------------------------

-- | Fail, providing a reference to the place in the code where
-- this function is called.
--
-- Like 'error' in Haskell code, this instruction is for internal errors only.
failUnexpected :: MText -> s :-> t
failUnexpected msg = failUsing $ [mt|Unexpected: |] <> msg

----------------------------------------------------------------------------
-- Custom errors
----------------------------------------------------------------------------

{- | Declares a custom error, defining @error name - error argument@ relation.

If your error is supposed to carry no argument, then provide @()@.

Note that this relation is defined globally rather than on per-contract basis,
so define errors accordingly. If your error has argument specific to your
contract, call it such that error name reflects its belonging to this contract.

This is the basic [error format].
-}

{- About global registry of errors:

Pros: this allows defining pieces of code which can be reused among the contracts.

Cons: possible name collisions. Example of worst case: two libraries define
different errors for the same tag, attempt to use both of these libraries would
cause compile-time error. But such infrastructure will barely take place in
practice.
-}
{- For future work:

* Assosiating unique numeric error codes to tags, and put codes into
the contract instead of tags. This allows getting rid of overhead related to
often long tag literals.

* Allow using arbitrary type instead of tag (and 'KnownSymbol' constraint will
be replaced with a more generic one).
Some users may prefer using datatypes as tags rather than type-level strings as tags
because
  * there will be no orphan instances then
  * no problem with name collisions
  * compiler will prompt a hint in case of typo in tag name
  * in some cases autocomplete handles declarations in code better than strings
(and cons here is that each such tag will need to be declared first).

-}
type family ErrorArg (tag :: Symbol) :: Type

-- | Material custom error.
--
-- Use this in pattern matches against error (e.g. in tests).
data CustomError (tag :: Symbol) = CustomError
  { ceTag :: Label tag
  , ceArg :: CustomErrorRep tag
  }

deriving stock instance Eq (CustomErrorRep tag) => Eq (CustomError tag)
deriving stock instance Show (CustomErrorRep tag) => Show (CustomError tag)

instance Buildable (CustomError tag) where
  build (CustomError tg _err) = "CustomError #" +| build tg

-- | To be used as @ErrorArg@ instance when failing with just
-- a @string@ instead of @pair string x@
data NoErrorArg

-- | To be used as @ErrorArg@ instances. This is equivalent to using
-- @()@ but using @UnitErrorArg@ is preferred since @()@ behavior could
-- be changed in the future.
data UnitErrorArg

-- | How 'CustomError' is actually represented in Michelson.
type CustomErrorRep tag = CustomErrorArgRep (ErrorArg tag)

type family CustomErrorArgRep (errArg :: Type) where
  CustomErrorArgRep NoErrorArg = MText
  CustomErrorArgRep UnitErrorArg = (MText, ())
  CustomErrorArgRep errArg = (MText, errArg)

-- | Typeclass implements various method that work with `CustomErrorArgRep`.
class IsCustomErrorArgRep a where
  verifyErrorTag :: MText -> a -> Either Text a
  customErrorRepDocDeps :: [SomeDocDefinitionItem]
  customErrorHaskellRep
    :: (KnownSymbol tag, CustomErrorHasDoc tag)
    => Proxy tag -> Markdown

instance IsCustomErrorArgRep MText where
  verifyErrorTag expectedTag tag =
    if tag == expectedTag
    then Right tag
    else Left $ "Bad tag, expected " +| expectedTag |+ ", got " +| tag |+ ""
  customErrorRepDocDeps = []
  customErrorHaskellRep (_ :: Proxy tag) =
    mdTicked $ build (errorTagToText @tag)

instance (TypeHasDoc errArg)
  => IsCustomErrorArgRep (MText, errArg) where
  verifyErrorTag expectedTag (tag, arg) =
    if tag == expectedTag
    then Right (tag, arg)
    else Left $ "Bad tag, expected " +| expectedTag |+ ", got " +| tag |+ ""
  customErrorRepDocDeps =
    [ SomeDocDefinitionItem $ DType $ Proxy @errArg ]
  customErrorHaskellRep (_ :: Proxy tag) = mconcat
    [ mdTicked ("(" <> build (errorTagToText @tag) <> ", " <> "<error argument>" <> ")")
    , ("\n\nProvided error argument will be of type "
      <> typeDocMdReference (Proxy @(MText, errArg)) (WithinParens False)
      <> (maybe "" (\txt -> " and stand for " <> txt) (customErrArgumentSemantics @tag))
      <> "."
      )
    ]

-- | This instance cannot be implemented, use 'IsError' instance instead.
instance (WellTypedIsoValue (CustomErrorRep tag), TypeError ('Text "CustomError has no IsoValue instance")) =>
         IsoValue (CustomError tag) where
  -- Originally we had a `TypeError` here, but that had to be changed when
  -- `WellTyped (ToT a)` was added as a superclass of `IsoValue`, because it
  -- resulted in the type error being triggerred from the evaluation of `ToT`
  -- typefamily in the super class clause.
  type ToT (CustomError tag) = (ToT (CustomErrorRep tag))
  toVal = error "impossible"
  fromVal = error "impossible"

instance ( CustomErrorHasDoc tag
         , KnownError (CustomErrorRep tag)
         , IsoValue (CustomErrorRep tag)
         , IsCustomErrorArgRep (CustomErrorRep tag)
         )
  => IsError (CustomError tag) where

  errorToVal (CustomError _ arg) cont =
    cont $ toVal @(CustomErrorRep tag) arg

  errorFromVal v = do
    let expectedTag = errorTagToMText (fromLabel @tag)
    v' <- castM v \MkMismatchError{..} -> Left $ "Wrong type for custom error: " <> pretty meActual
    errArg <- verifyErrorTag @(CustomErrorRep tag) expectedTag
            $ fromVal @(CustomErrorRep tag) v'
    pure $ CustomError fromLabel errArg

instance ( CustomErrorHasDoc tag
         , IsCustomErrorArgRep (CustomErrorRep tag)
         )
  => ErrorHasDoc (CustomError tag) where
  errorDocName = errorTagToText @tag
  errorDocDependencies = customErrorRepDocDeps @(CustomErrorRep tag)
  errorDocMdCause = customErrDocMdCause @tag
  errorDocMdCauseInEntrypoint = customErrDocMdCauseInEntrypoint @tag
  errorDocClass = customErrClass @tag
  errorDocHaskellRep = customErrorHaskellRep @(CustomErrorRep tag) (Proxy @tag)

  type ErrorRequirements (CustomError tag) = (CustomErrorHasDoc tag, IsCustomErrorArgRep (CustomErrorRep tag))
  errorDocRequirements = Dict

-- | Demote error tag to term level.
errorTagToMText :: Label tag -> MText
errorTagToMText l =
  -- Now tags come not from constructor names, but from labels,
  -- we have to lead the first letter to upper case to preserve
  -- compatibility with FA1.2 interface.
  mtextHeadToUpper $
    labelToMText l

errorTagToText :: forall tag. KnownSymbol tag => Text
errorTagToText = toText $ errorTagToMText (fromLabel @tag)

-- | Fail with given custom error.
failCustom
  :: forall tag err s any.
     ( MustHaveErrorArg tag (MText, err)
     , CustomErrorHasDoc tag
     , KnownError err
     )
  => Label tag -> err : s :-> any
failCustom l =
  doc (DThrows (Proxy @(CustomError tag))) #
  push (errorTagToMText l) # pair @MText @err #
  FI (FAILWITH @(ToT (MText, err)))

-- | Fail with given custom error.
failCustomNoArg
  :: forall tag s any.
     ( MustHaveErrorArg tag MText
     , CustomErrorHasDoc tag
     )
  => Label tag -> s :-> any
failCustomNoArg l =
  doc (DThrows (Proxy @(CustomError tag))) #
  push (errorTagToMText l) #
  FI (FAILWITH @(ToT (MText)))


type MustHaveErrorArg errorTag expectedArgRep =
  ( AssertTypesEqual (CustomErrorRep errorTag) expectedArgRep
      ('Text "Error argument type is "
        ':<>: 'ShowType (expectedArgRep)
        ':<>: 'Text " but given error requires argument of type "
        ':<>: 'ShowType (CustomErrorRep errorTag)
      )
  , CustomErrorRep errorTag ~ expectedArgRep
  )

-- | Specialization of 'failCustom' for unit-arg errors.
failCustom_
  :: forall tag s any.
     ( MustHaveErrorArg tag (MText, ())
     , CustomErrorHasDoc tag
     )
  => Label tag -> s :-> any
failCustom_ l =
  reifyTypeEquality @(CustomErrorRep tag) @(MText, ()) $
    doc (DThrows (Proxy @(CustomError tag))) #
    unit #
    push (errorTagToMText l) # pair @MText @() #
    FI (FAILWITH @(ToT (MText, ())))

-- Special treatment of no-arg errors
----------------------------------------------------------------------------

instance Eq (CustomErrorRep tag) => Eq (() -> CustomError tag) where
  e1 == e2 = e1 () == e2 ()
instance Show (CustomErrorRep tag) => Show (() -> CustomError tag) where
  show e = Debug.show (e ())

-- | If 'CustomError' constructor is not provided its argument, we assume
-- that this is unit-arg error and interpret the passed value as complete.
instance ( Typeable arg
         , IsError (CustomError tag)
         , AssertTypesEqual arg () notVoidError
         , arg ~ ErrorArg tag
         , notVoidError ~
            ('Text "This error requires argument of type "
              ':<>: 'ShowType (ErrorArg tag)
            )
         ) =>
         IsError (arg -> CustomError tag) where
  errorToVal mkCustomError cont =
    reifyTypeEquality @arg @() $
      errorToVal (mkCustomError ()) cont
  errorFromVal v =
    reifyTypeEquality @arg @() $
      errorFromVal v <&> \(CustomError l a) b -> CustomError l (fst a, b)

instance (Typeable arg, ErrorHasDoc (CustomError tag)) =>
         ErrorHasDoc (arg -> CustomError tag) where
  errorDocName = errorDocName @(CustomError tag)
  errorDocMdCauseInEntrypoint = errorDocMdCauseInEntrypoint @(CustomError tag)
  errorDocMdCause = errorDocMdCause @(CustomError tag)
  errorDocHaskellRep = errorDocHaskellRep @(CustomError tag)
  errorDocDependencies = errorDocDependencies @(CustomError tag)

----------------------------------------------------------------------------
-- Errors documentation injection into contracts
----------------------------------------------------------------------------

-- | Error class on how the error should be handled by the client.
data ErrorClass
  = ErrClassActionException
    -- ^ Normal expected error.
    -- Examples: "insufficient balance", "wallet does not exist".
  | ErrClassBadArgument
    -- ^ Invalid argument passed to entrypoint.
    -- Examples: your entrypoint accepts an enum represented as @nat@, and
    -- unknown value is provided.
    -- This includes more complex cases which involve multiple entrypoints.
    -- E.g. API provides iterator interface, middleware should care about using it
    -- hiding complex details and exposing a simpler API to user; then an attempt to
    -- request non-existing element would also correspond to an error from this class.
  | ErrClassContractInternal
    -- ^ Unexpected error. Most likely it means that there is a bug in
    -- the contract or the contract has been deployed incorrectly.
  | ErrClassUnknown
    -- ^ It's possible to leave error class unspecified.
  deriving stock (Lift)

instance Read ErrorClass where
  readsPrec _ = \case
    "exception" -> [(ErrClassActionException, "")]
    "bad-argument" -> [(ErrClassBadArgument, "")]
    "contract-internal" -> [(ErrClassContractInternal, "")]
    "unknown" -> [(ErrClassUnknown, "")]
    _ -> []

instance Buildable ErrorClass where
  build = \case
    ErrClassActionException -> "Action exception"
    ErrClassBadArgument -> "Bad argument"
    ErrClassContractInternal -> "Internal"
    ErrClassUnknown -> "-"

-- | Whether given error class is about internal errors.
--
-- Internal errors are not enlisted on per-entrypoint basis, only once for
-- the entire contract.
isInternalErrorClass :: ErrorClass -> Bool
isInternalErrorClass = \case
  ErrClassActionException -> False
  ErrClassBadArgument -> False
  ErrClassContractInternal -> True
  ErrClassUnknown -> False

class (KnownSymbol tag, TypeHasDoc (CustomErrorRep tag), IsError (CustomError tag)) =>
      CustomErrorHasDoc tag where
  -- | What should happen for this error to be raised.
  customErrDocMdCause :: Markdown

  -- | Brief version of 'customErrDocMdCause'.
  -- This will appear along with the error when mentioned in entrypoint description.
  --
  -- By default, the first sentence of the full description is used.
  customErrDocMdCauseInEntrypoint :: Markdown
  customErrDocMdCauseInEntrypoint = pickFirstSentence $ customErrDocMdCause @tag

  -- | Error class.
  --
  -- By default this returns "unknown error" class; though you should provide
  -- explicit implementation in order to avoid a warning.
  customErrClass :: ErrorClass
  customErrClass = ErrClassUnknown

  -- | Clarification of error argument meaning.
  --
  -- Provide when it's not obvious, e.g. argument is not named with t'Lorentz.ADT.:!'.
  --
  -- NOTE: This should /not/ be an entire sentence, rather just the semantic
  -- backbone.
  --
  -- Bad:
  -- * @Error argument stands for the previous value of approval.@
  --
  -- Good:
  -- * @the previous value of approval@
  -- * @pair, first argument of which is one thing, and the second is another@
  customErrArgumentSemantics :: Maybe Markdown
  customErrArgumentSemantics = Nothing
  {-# MINIMAL customErrDocMdCause, customErrClass #-}

-- | Mentions that contract uses given error.
data DError where
  DError :: ErrorHasDoc e => Proxy e -> DError

instance Eq DError where
  DError e1 == DError e2 = e1 `eqParam1` e2
instance Ord DError where
  DError e1 `compare` DError e2 = e1 `compareExt` e2

instance DocItem DError where
  type DocItemPlacement DError = 'DocItemInDefinitions
  type DocItemReferenced DError = 'True

  docItemPos = 5010
  docItemSectionName = Just "Errors"
  docItemSectionDescription = Just errorsDocumentation
  docItemRef (DError (_ :: Proxy e)) = DocItemRef $
    DocItemId ("errors-" <> errorDocName @e)
  docItemToMarkdown lvl (DError (_ :: Proxy e)) =
    mconcat
    [ mdSeparator
    , mdHeader lvl (mdTicked . build $ errorDocName @e)
    , mdSubsection "Class" (build $ errorDocClass @e)
    , "\n\n"
    , mdSubsection "Fires if" $ errorDocMdCause @e
    , "\n\n"
    , mdSubsection "Representation" $ errorDocHaskellRep @e
    ]
  docItemToToc lvl d@(DError (_ :: Proxy e)) =
    mdTocFromRef lvl (build $ errorDocName @e) d

  docItemDependencies (DError (_ :: Proxy e)) = errorDocDependencies @e

errorDocMdReference :: forall e. ErrorHasDoc e => Markdown
errorDocMdReference =
  let DocItemRef docItemId = docItemRef $ DError (Proxy @e)
  in mdLocalRef (mdTicked . build $ errorDocName @e) docItemId

-- | Documentation for custom errors.

-- | Mentions that entrypoint throws given error.
data DThrows where
  DThrows :: ErrorHasDoc e => Proxy e -> DThrows

instance Eq DThrows where
  DThrows e1 == DThrows e2 = eqParam1 e1 e2

instance DocItem DThrows where
  docItemPos = 5011
  docItemSectionName = Just "Possible errors"
  docItemSectionNameStyle = DocSectionNameSmall
  docItemDependencies (DThrows ds) =
    [SomeDocDefinitionItem (DError ds)]
  docItemToMarkdown _ (DThrows (_ :: Proxy e)) =
    "* " <> errorDocMdReference @e <> " — " <> errorDocMdCauseInEntrypoint @e
  docItemsOrder =
    let errType (DThrows (_ :: Proxy e)) = errorDocClass @e
    in L.nub . filter (Prelude.not . isInternalErrorClass . errType)

-- | Implementation of 'typeDocMdDescription' (of 'TypeHasDoc' typeclass)
-- for Haskell types which sole purpose is to be error.
typeDocMdDescriptionReferToError :: forall e. IsError e => Markdown
typeDocMdDescriptionReferToError =
  "This type is primarily used as error, see " <>
  docDefinitionRef "description in section with errors" (DError (Proxy @e))

-- | This is to be included on top of @Errors@ section of the generated
-- documentation.
errorsDocumentation :: Markdown
errorsDocumentation =
  -- Note: this description should remain general enough to fit into all our
  -- error formats, seek for [error format] to find all the relevant
  -- declarations.
  [md|
  Our contract implies the possibility of error scenarios, this section enlists
  all values which the contract can produce via calling `FAILWITH` instruction
  on them. In case of error, no changes to contract state will be applied.

  Each entrypoint also contains a list of errors which can be raised during its
  execution; only for no-throw entrypoints this list will be omitted.
  Errors in these lists are placed in the order in which the corresponding
  properties are checked unless the opposite is specified. I.e., if for a
  given entrypoint call two different errors may take place, the one which
  appears in the list first will be thrown.

  The errors are represented either as a string `error tag` or a pair `(error tag, error argument)`.
  See the list of errors below for details.

  We distinquish several error classes:
  + #{errClassActionException}: given action cannot be performed with
    regard to the current contract state.

    Examples: "insufficient balance", "wallet does not exist".

    If you are implementing a middleware, such errors should be propagated to
    the client.

  + #{errClassBadArgument}: invalid argument supplied to the entrypoint.

    Examples: entrypoint accepts a natural number from `0-3` range, and you
    supply `5`.

    If you are implementing a middleware, you should care about not letting
    such errors happen.

  + #{errClassContractInternal}: contract-internal error.

    In ideal case, such errors should not take place, but still, make sure
    that you are ready to handle them. They can signal either invalid contract
    deployment or a bug in contract implementation.

    If an internal error is thrown, please report it to the author of this contract.
  |]
  where
    errClassActionException  = mdBold $ build ErrClassActionException
    errClassBadArgument      = mdBold $ build ErrClassBadArgument
    errClassContractInternal = mdBold $ build ErrClassContractInternal