Safe Haskell	None
Language	Haskell2010

Data.RBR.Examples

Contents

Setup code
Constructing a record and viewing its fields.
Getting a subset of fields out of a record
Creating a Record out of a conventional Haskell record
Injecting into a Variant and eliminating it
Working with a bigger error type inside a function
Creating a Variant out of a sum type and matching on it
Changing the way a specific record field is parsed from JSON
Parsing a record from JSON using aliased fields
Parsing a subset of a record's fields from JSON and inserting them in an existing record value
Ensuring all branches of a sum type are parsed from JSON
External examples

Synopsis

Setup code

>>> :set -XDataKinds -XTypeApplications
>>> :set -XFlexibleContexts -XTypeFamilies -XAllowAmbiguousTypes -XScopedTypeVariables
>>> :set -XDeriveGeneric
>>> :set -XPartialTypeSignatures
>>> :set -Wno-partial-type-signatures
>>> import Data.RBR
>>> import Data.SOP
>>> import Data.SOP.NP (cpure_NP,sequence_NP,liftA2_NP,collapse_NP)
>>> import Data.String
>>> import Data.Proxy
>>> import Data.Foldable
>>> import Data.Profunctor (Star(..))
>>> import GHC.Generics
>>> import qualified Data.Text
>>> import Data.Aeson
>>> import Data.Aeson.Types (explicitParseField,Parser,parseMaybe)

Constructing a record and viewing its fields.

We use addFieldI instead of addField because we are dealing with pure records.

>>> :{
    let r = addFieldI @"name" "Foo"
          . addFieldI @"age"  5
          $ unit
     in print (getFieldI @"name" r)
:}
"Foo"

Getting a subset of fields out of a record

Notice that the subset is specified as a type-level tree using FromList, a type family that takes a list of type-level tuples.

Because here the types of each field can be inferred, we can use a wildcard (enabled by the PartialTypeSignatures extension).

>>> :{
    let r = addFieldI @"name"      "Foo"
          . addFieldI @"age"       5
          . addFieldI @"whatever"  'x'
          $ unit
        s = getFieldSubset @(FromList [ '("age",_), '("whatever",_) ]) r
     in putStrLn (prettyShowRecordI s)
:}
{age = 5, whatever = 'x'}

Creating a Record out of a conventional Haskell record

>>> data Person = Person { name :: String, age :: Int } deriving (Generic, Show)
>>> instance ToRecord Person
>>> :{
    let r = addFieldI @"whatever" 'x' (toRecord (Person "Foo" 50))
     in putStrLn (prettyShowRecordI r)
:}
{age = 50, name = "Foo", whatever = 'x'}

Injecting into a Variant and eliminating it

Here the full type of the Variant is inferred from the type of its Record of eliminators.

>>> :{
    let b = injectI @"left" 'c'
        e = addCaseI @"left" putChar
          . addCaseI @"right" @Bool print
          $ unit
     in eliminate e b
:}
c

Working with a bigger error type inside a function

A function can use internally an error Variant bigger than the one it eventually returns. The internal branches of the Variant can be removed with winnow.

This library makes it more involved than it should be, because inserting an entry and then deleting it can result in structurally dissimilar type-level maps. So we need extra type annotations in winnow, and also a call to injectSubset to perform the conversion.

>>> type Smaller = FromList '[ '("foo",Char), '("bar",Int) ]
>>> :{
    let func :: Int -> Variant I Smaller 
        func i = 
            let v = if (i == 0) then injectI @"baz" "internal"
                                else injectI @"foo" 'c'
                r = case winnowI @"baz" @String @(Insert "baz" String Smaller) v of
                        Right   e       -> error "this is the baz internal error"
                        Left    smaller -> smaller
             in injectSubset r
     in putStrLn $ prettyShowVariantI (func 1)
:}
foo ('c')

Creating a Variant out of a sum type and matching on it

>>> data Summy = Lefty Int | Righty Bool deriving (Generic,Show)
>>> instance ToVariant Summy
>>> :{
    let v = toVariant (Lefty 5)
     in matchI @"Lefty" v
:}
Just 5

Changing the way a specific record field is parsed from JSON

We start in the sop-core world, creating a product of parsing functions (one for each field) using cpure_NP.

Then we convert that product to a Record, apply to it a transformation that uses field selectors, and convert it back to a product.

Then we demote the field names and combine them with the product of Value parsers using liftA2_NP, getting a product of Object parsers.

Then we use sequence_NP to convert the product of parsers into a parser of Record.

>>> :{
    let parseSpecial
              :: forall r c flat. (Generic r, 
                                   FromRecord r, 
                                   RecordCode r ~ c, 
                                   KeysValuesAll KnownKey c, 
                                   Productlike '[] c flat, 
                                   All FromJSON flat) 
              => (Record (Star Parser Data.Aeson.Value) c -> Record (Star Parser Data.Aeson.Value) c)
              -> Data.Aeson.Value 
              -> Parser r
        parseSpecial transform = 
            let mapKSS (K name) (Star pf) = Star (\o -> explicitParseField pf o (Data.Text.pack name))
                fieldParsers = transform $ fromNP @c (cpure_NP (Proxy @FromJSON) (Star parseJSON))
                Star parser = fromNP <$> sequence_NP (liftA2_NP mapKSS (toNP @c demoteKeys) (toNP fieldParsers))
             in withObject "someobj" $ \o -> fromRecord <$> parser o
    :}

>>> data Person = Person { name :: String, age :: Int } deriving (Generic, Show)
>>> instance ToRecord Person
>>> instance FromRecord Person
>>> :{
    instance FromJSON Person where 
        parseJSON = parseSpecial (setField @"name" (Star (\_ -> pure "foo")))
    :}

>>> Data.Aeson.eitherDecode @Person (fromString "{ \"name\" : null, \"age\" : 50 }")
Right (Person {name = "foo", age = 50})

Parsing a record from JSON using aliased fields

The aliases are passed as a Record with values wrapped in the K functor. This means that there aren't really any values of the type that corresponds to each field, only the String annotations.

>>> :{
    let parseWithAliases
              :: forall r c flat. (Generic r, 
                                   FromRecord r, 
                                   RecordCode r ~ c, 
                                   KeysValuesAll KnownKey c, 
                                   Productlike '[] c flat, 
                                   All FromJSON flat) 
              => Record (K String) c
              -> Data.Aeson.Value 
              -> Parser r
        parseWithAliases aliases = 
            let mapKSS (K name) (Star pf) = Star (\o -> explicitParseField pf o (Data.Text.pack name))
                fieldParsers = cpure_NP (Proxy @FromJSON) (Star parseJSON)
                Star parser = fromNP <$> sequence_NP (liftA2_NP mapKSS (toNP @c aliases) fieldParsers)
             in withObject "someobj" $ \o -> fromRecord <$> parser o
    :}

We have to use getFieldSubset because the aliases are listed in a different order than the record fields, and that might result in different type-level trees. If the orders were the same, we wouldn't need it.

>>> data Person = Person { name :: String, age :: Int } deriving (Generic, Show)
>>> instance ToRecord Person
>>> instance FromRecord Person
>>> :{
    instance FromJSON Person where 
        parseJSON = let aliases = addField @"age"  (K "bar")
                                . addField @"name" (K "foo")
                                $ unit
                     in parseWithAliases (getFieldSubset @(RecordCode Person) aliases)
    :}

>>> Data.Aeson.eitherDecode @Person (fromString "{ \"foo\" : \"John\", \"bar\" : 50 }")
Right (Person {name = "John", age = 50})

Parsing a subset of a record's fields from JSON and inserting them in an existing record value

>>> :{
    let parseFieldSubset
              :: forall subset subflat c flat r. (Generic r, 
                                                  FromRecord r, 
                                                  RecordCode r ~ c, 
                                                  ProductlikeSubset subset c subflat,
                                                  KeysValuesAll KnownKey subset, 
                                                  All FromJSON subflat) 
              => r 
              -> Data.Aeson.Value
              -> Parser r 
        parseFieldSubset r = 
            let mapKSS (K name) (Star pf) = Star (\o -> explicitParseField pf o (Data.Text.pack name))
                objNP = liftA2_NP mapKSS (toNP @subset demoteKeys) (cpure_NP (Proxy @FromJSON) (Star parseJSON)) 
                intoOriginal subr = fromRecord (setFieldSubset @subset subr (toRecord r))
                Star subparser = intoOriginal . fromNP @subset <$> sequence_NP objNP
             in withObject "someobj" subparser
    :}

>>> data Person = Person { name :: String, age :: Int, whatever :: Bool } deriving (Generic, Show)
>>> instance ToRecord Person
>>> instance FromRecord Person
>>> :{
    let original = Person "John" 50 True
        Just v = Data.Aeson.decode @Data.Aeson.Value (fromString "{ \"name\" : \"Mark\", \"age\" : 70 }")
        subsetParser = parseFieldSubset @(FromList [ '("name",_), '("age",_) ]) original
        Just s = parseMaybe subsetParser v
     in s
    :}
Person {name = "Mark", age = 70, whatever = True}

Ensuring all branches of a sum type are parsed from JSON

To ensure that we don't forget any branch when parsing a sum type from JSON, we can create a n-ary product of parsers, one for each branch.

Then we create a n-ary product of injections. Each component of the product creates a n-ary sum out of the value of the corresponding branch.

We combine the n-ary product of parsers with the n-ary product of injections, and collapse all the resulting parsers with asum.

Then we convert the n-ary sum value that "wins" into a Variant and finally back into the original type.

>>> :{
    let parseAll
              :: forall r c flat. (Generic r, 
                                   FromVariant r, 
                                   VariantCode r ~ c, 
                                   KeysValuesAll KnownKey c, 
                                   Productlike '[] c flat, 
                                   Sumlike '[] c flat, 
                                   All FromJSON flat) 
              => Data.Aeson.Value 
              -> Parser r
        parseAll = 
            let mapKSS (K name) (Star pf) = Star (\o -> explicitParseField pf o (Data.Text.pack name))
                branchParsers = liftA2_NP mapKSS (toNP @c demoteKeys) (cpure_NP (Proxy @FromJSON) (Star parseJSON))
                injected = liftA2_NP (\f star -> K (unK . apFn f . I <$> star)) (injections @flat) branchParsers 
                Star parser = asum $ collapse_NP injected
             in withObject "someobj" (\o -> fromVariant @r . fromNS <$> parser o)
    :}

>>> data ThisOrThat = This String | That Int deriving (Generic, Show)
>>> instance FromVariant ThisOrThat
>>> :{
    let Just v = Data.Aeson.decode @Data.Aeson.Value (fromString "{ \"That\" : 70 }")
        Just s = parseMaybe (parseAll @ThisOrThat) v
     in s
    :}
That 70