https://gitlab.com/igrep/deriveJsonNoPrefix

Template Haskell macros to derive ToJSON/FromJSON instances in a more prefix-friendly manner.
https://gitlab.com/igrep/deriveJsonNoPrefix

haskell json template haskell

Last synced: about 1 month ago
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Template Haskell macros to derive ToJSON/FromJSON instances in a more prefix-friendly manner.

• Host: gitlab.com
• URL: https://gitlab.com/igrep/deriveJsonNoPrefix
• Owner: igrep
• License: apache-2.0
• Created: 2018-07-16T05:32:34.313Z (over 6 years ago)
• Default Branch: master
• Last Synced: 2024-04-25T23:31:23.299Z (7 months ago)
• Topics: haskell, json, template haskell
• Stars: 0
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: ChangeLog.md
  • License: LICENSE

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README

        
# deriveJsonNoPrefix

Template Haskell macros to derive ToJSON/FromJSON instances in a more prefix-friendly manner.

## Example

Suppose you want to create a JSON like this:

```json

{

    "id": "ID STRING",

    "max": 0.789,

    "min": 0.123

}

```

You'd want to define a record type to derive the instance of [ToJSON](http://hackage.haskell.org/package/aeson/docs/Data-Aeson.html#t:ToJSON) (and possibly [FromJSON](http://hackage.haskell.org/package/aeson/docs/Data-Aeson.html#t:FromJSON)) automatically:

```hs

{-# LANGUAGE TemplateHaskell #-}

import Data.Aeson.TH

data SomeRecord = SomeRecord

  { id :: String

  , max :: Double

  , min :: Double

  } deriving (Eq, Show)

$(deriveToJSON ''SomeRecord)

```

But you shouldn't define such a record because both `id`, `max`, and `min` are predefined functions of `Prelude`!!

As a workaround, we frequently prefix the record labels with their type name:

```hs

data SomeRecord = SomeRecord

  { someRecordId :: String

  , someRecordMax :: Double

  , someRecordMin :: Double

  } deriving (Eq, Show)

```

Then `deriveToJSON` with a modified option:

```hs

deriveToJSON Json.defaultOptions { fieldLabelModifier = firstLower . drop (length "SomeRecord") } ''SomeRecord

firstLower :: String -> String

firstLower (x:xs) = toLower x : xs

firstLower _ = error "firstLower: Assertion failed: empty string"

```

That's almost exactly what `deriveToJsonNoTypeNamePrefix` does!!  

`deriveToJsonNoTypeNamePrefix` is essentially defined as:

```hs

deriveToJsonNoTypeNamePrefix :: Name -> Q [Dec]

deriveToJsonNoTypeNamePrefix deriver name =

  deriveToJSON Json.defaultOptions { fieldLabelModifier = dropPrefix name } name

dropPrefix :: Name -> String -> String

dropPrefix name = firstLower . drop (length $ nameBase name)

firstLower :: String -> String

firstLower (x:xs) = toLower x : xs

firstLower _ = error "firstLower: Assertion failed: empty string"

```

So now, you don't have reimplement the `fieldLabelModifier` anymore!

```hs

import Data.Aeson.DeriveNoPrefix

$(deriveJsonNoTypeNamePrefix ''SomeRecord)

```

## Other libraries which would solve the same problem

- [extensible](https://hackage.haskell.org/package/extensible).

- And other libraries providing extensible records with `ToJSON` / `FromJSON` instances.

So use this package all of them are too heavy in learning cost / dependency footprint / etc.