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This introduces an `ErrorMessage` newtype which wraps `Text` in a manner which is designed to be easy to construct, and difficult to deconstruct. It provides functionality similar to `Data.Text.Extended`, but designed _only_ for error messages. Error messages are constructed through `fromString`, concatenation, or the `toErrorValue` function, which is designed to be overridden for all meaningful domain types that might show up in an error message. Notably, there are not and should never be instances of `ToErrorValue` for `String`, `Text`, `Int`, etc. This is so that we correctly represent the value in a way that is specific to its type. For example, all `Name` values (from the _graphql-parser-hs_ library) are single-quoted now; no exceptions. I have mostly had to add `instance ToErrorValue` for various backend types (and also add newtypes where necessary). Some of these are not strictly necessary for this changeset, as I had bigger aspirations when I started. These aspirations have been tempered by trying and failing twice. As such, in this changeset, I have started by introducing this type to the `parseError` and `parseErrorWith` functions. In the future, I would like to extend this to the `QErr` record and the various `throwError` functions, but this is a much larger task and should probably be done in stages. For now, `toErrorMessage` and `fromErrorMessage` are provided for conversion to and from `Text`, but the intent is to stop exporting these once all error messages are converted to the new type. PR-URL: https://github.com/hasura/graphql-engine-mono/pull/5018 GitOrigin-RevId: 84b37e238992e4312255a87ca44f41af65e2d89a
47 lines
1.5 KiB
Haskell
47 lines
1.5 KiB
Haskell
-- | Error messages
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--
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-- This module defines a type for user facing error messages.
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--
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-- To construct a value of this type, use `toErrorMessage` or the 'IsString' interface,
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-- the type class 'ToErrorValue' defined in the "Hasura.Base.ToErrorValue" module,
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-- or use the utility functions defined in the "Hasura.Base.ErrorValue" module.
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--
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-- 'ErrorMessage's can also be composed using the 'Semigroup' interface.
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--
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-- To fail with an error message in monadic code, use the 'failWithMessage' function.
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module Hasura.Base.ErrorMessage
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( ErrorMessage,
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ErrorMessageOr,
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toErrorMessage,
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fromErrorMessage,
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failWithMessage,
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)
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where
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import Data.Aeson
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import Data.String (IsString (..))
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import Data.Text qualified as Text
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import Hasura.Prelude
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-- | 'ErrorMessage' wraps a 'Text' value such that it's easy to build up,
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-- but difficult to break apart or extract the underlying text value.
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newtype ErrorMessage = ErrorMessage
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{ -- | A temporary extractor which will go away once 'ErrorMessage' is pervasive.
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fromErrorMessage :: Text
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}
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deriving newtype (Eq, Semigroup, ToJSON)
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-- | A smart constructor for 'ErrorMessage' so that it cannot be deconstructed.
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toErrorMessage :: Text -> ErrorMessage
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toErrorMessage = ErrorMessage
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-- | Fails with the given message.
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failWithMessage :: MonadFail m => ErrorMessage -> m ()
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failWithMessage = fail . Text.unpack . fromErrorMessage
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instance IsString ErrorMessage where
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fromString = ErrorMessage . Text.pack
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-- | A simple alias to prevent duplication.
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type ErrorMessageOr = Either ErrorMessage
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