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Haskell binding for Gremlin graph query language
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greskell - Haskell binding for Gremlin graph query language

greskell is a toolset to build and execute Gremlin graph query language in Haskell.



Because this README is also a test script, first we import common modules.

{-# LANGUAGE OverloadedStrings, QuasiQuotes, TypeFamilies #-}
import Control.Category ((>>>))
import Control.Monad (guard)
import Data.Monoid (mempty)
import Data.Text (Text)
import qualified Data.HashMap.Strict as HM
import qualified Data.Aeson as A
import qualified Data.Aeson.Types as A
import Data.Function ((&))
import Text.Heredoc (here)
import Test.Hspec

To run the examples in this README, run stack test test-readme. See test-readme directory to see how this works.

The Greskell type

At the core of greskell is the Greskell type. Greskell a represents a Gremlin expression that evaluates to the type a.

import Data.Greskell.Greskell (Greskell, toGremlin)

literalText :: Greskell Text
literalText = "foo"

literalInt :: Greskell Int
literalInt = 200

You can convert Greskell into Gremlin Text script by toGremlin function.

main = hspec $ specify "Greskell" $ do
  toGremlin literalText `shouldBe` "\"foo\""

Greskell implements instances of IsString, Num, Fractional etc. so you can use methods of these classes to build Greskell.

  toGremlin (literalInt + 30 * 20) `shouldBe` "(200)+((30)*(20))"

Build variable binding

Gremlin Server supports parameterized scripts, where a client can send a Gremlin script and variable binding.

greskell's Binder monad is a simple monad that manages bound variables and their values. With Binder, you can inject Haskell values into Greskell.

import Data.Greskell.Greskell (Greskell, toGremlin)
import Data.Greskell.Binder (Binder, newBind, runBinder)

plusTen :: Int -> Binder (Greskell Int)
plusTen x = do
  var_x <- newBind x
  return $ var_x + 10

newBind creates a new Gremlin variable unique in the Binder's monadic context, and returns that variable.

main = hspec $ specify "Binder" $ do
  let (script, binding) = runBinder $ plusTen 50
  toGremlin script `shouldBe` "(__v0)+(10)"
  binding `shouldBe` HM.fromList [("__v0", A.Number 50)]

runBinder function returns the Binder's monadic result and the created binding.

Submit to the Gremlin Server

To connect to the Gremlin Server and submit your Gremlin script, use greskell-websocket package.

import Control.Exception.Safe (bracket, try, SomeException)
import Data.Foldable (toList)
import Data.Greskell.Greskell (Greskell) -- from greskell package
import Data.Greskell.Binder -- from greskell package
  (Binder, newBind, runBinder)
import Network.Greskell.WebSocket -- from greskell-websocket package
  (connect, close, submit, slurpResults)
import System.IO (hPutStrLn, stderr)

submitExample :: IO [Int]
submitExample =
  bracket (connect "localhost" 8182) close $ \client -> do
    let (g, binding) = runBinder $ plusTen 50
    result_handle <- submit client g (Just binding)
    fmap toList $ slurpResults result_handle

plusTen :: Int -> Binder (Greskell Int)
plusTen x = do
  var_x <- newBind x
  return $ var_x + 10

main = hspec $ specify "submit" $ do
  egot <- try submitExample :: IO (Either SomeException [Int])
  case egot of
    Left e -> do
      hPutStrLn stderr ("submit error: " ++ show e)
      hPutStrLn stderr ("  We ignore the error. Probably there's no server running?")
    Right got -> do
      hPutStrLn stderr ("submit success: " ++ show got)
      got `shouldBe` [60]

submit function sends a Greskell to the server and returns a ResultHandle. ResultHandle is a stream of evaluation results returned by the server. slurpResults gets all items from ResultHandle.

DSL for graph traversals

greskell has a domain-specific language (DSL) for building Gremlin Traversal object. Two data types, GTraversal and Walk, are especially important in this DSL.

GTraversal is simple. It's just the greskell counterpart of GraphTraversal class in Gremlin.

Walk is a little tricky. It represents a chain of one or more method calls on a GraphTraversal object. In Gremlin, those methods are called "graph traversal steps." greskell defines those traversal steps as functions returning a Walk object.

For example,

import Data.Greskell.Greskell (toGremlin, Greskell)
import Data.Greskell.GTraversal
  ( GTraversal, Transform, Walk, source, sV,
    gHasLabel, gHas2, (&.), ($.)
import Data.Greskell.Graph (AVertex)

allV :: GTraversal Transform () AVertex
allV = source "g" & sV []

isPerson :: Walk Transform AVertex AVertex
isPerson = gHasLabel "person"

isMarko :: Walk Transform AVertex AVertex
isMarko = gHas2 "name" "marko"

main = hspec $ specify "GTraversal" $ do
  toGremlin (allV &. isPerson &. isMarko)

In the above example, allV is the GraphTraversal obtained by g.V(). isPerson and isMarko are method calls of .hasLabel and .has steps, respectively. (&.) operator combines a GTraversal and Walk to get an expression that the graph traversal steps are executed on the GraphTraversal.

The above example also uses AVertex type. AVertex is a type for a graph vertex. We will explain it in detail later in Graph structure types.

Note that we use (&) operator in the definition of allV. (&) operator from Data.Function module is just the flip of ($) operator. Likewise, greskell defines ($.) operator, so we could also write the above expression as follows.

  (toGremlin $ isMarko $. isPerson $. sV [] $ source "g")

Type parameters of GTraversal and Walk

GTraversal and Walk both have the same type parameters.

GTraversal walk_type start end
Walk       walk_type start end

GTraversal and Walk both take the traversers with data of type start, and emit the traversers with data of type end. We will explain walk_type later.

Walk is very similar to function (->). That is why it is an instance of Category, so you can compose Walks together. The example in the last section can also be written as

  let composite_walk = isPerson >>> isMarko
  toGremlin (source "g" & sV [] &. composite_walk)

Restrict effect of GTraversal by WalkType

The first type parameter of GTraversal and Walk is called "walk type". Walk type is a type marker to describe effect of the graph traversal. There are three walk types, Filter, Transform and SideEffect. All of them are instance of WalkType class.

  • Walks of Filter type do filtering only. It takes input traversers and emits some of them. It does nothing else. Example: .has and .filter steps.
  • Walks of Transform type may transform the input traversers but have no side effects. Example: .map and .out steps.
  • Walks of SideEffect type may alter the "side effect" context of the Traversal object or the state outside the Traversal object. Example: .aggregate and .addV steps.

Walk types are hierarchical. Transform is more powerful than Filter, and SideEffect is more powerful than Transform. You can "lift" a walk with a certain walk type to one with a more powerful walk type by liftWalk function.

import Data.Greskell.GTraversal
  ( Walk, Filter, Transform, SideEffect, GTraversal,
    liftWalk, source, sV, (&.),
    gHasLabel, gHas1, gAddV, gValues, gIdentity
import Data.Greskell.Graph (AVertex)
import Data.Greskell.Greskell (toGremlin)
import Network.Greskell.WebSocket (Client, ResultHandle, submit)

hasAge :: Walk Filter AVertex AVertex
hasAge = gHas1 "age"

hasAge' :: Walk Transform AVertex AVertex
hasAge' = liftWalk hasAge

Now what are these walk types useful for? Well, it allows you to build graph traversals in a safer way than you do with plain Gremlin.

In Haskell, we can distinguish pure and non-pure functions using, for example, IO monad. Likewise, we can limit power of traversals by using Filter or Transform walk types explicitly. That way, we can avoid executing unwanted side-effect accidentally.

nameOfPeople :: Walk Filter AVertex AVertex -> GTraversal Transform () Text
nameOfPeople pfilter =
  source "g" & sV [] &. gHasLabel "person" &. liftWalk pfilter &. gValues ["name"]

newPerson :: Walk SideEffect s AVertex
newPerson = gAddV "person"

main = hspec $ specify "liftWalk" $ do
  ---- This compiles
  toGremlin (nameOfPeople hasAge)
    `shouldBe` "g.V().hasLabel(\"person\").has(\"age\").values(\"name\")"

  ---- This doesn't compile.
  ---- It's impossible to pass a SideEffect walk to an argument that expects Filter.
  -- toGremlin (nameOfPeople newPerson)
  --   `shouldBe` "g.V().hasLabel(\"person\").addV(\"person\").values(\"name\")"

In the above example, nameOfPeople function takes a Filter walk and creates a Transform walk. There is no way to pass a SideEffect walk (like gAddV) to nameOfPeople because Filter is weaker than SideEffect. That way, we can be sure that the result traversal of nameOfPeople function never has any side-effect (thus its walk type is just Transform.)

Submit GTraversal

You can submit GTraversal directly to the Gremlin Server. Submitting GTraversal c s e yeilds ResultHandle e, so you can get the traversal results in a stream.

getNameOfPeople :: Client -> IO (ResultHandle Text)
getNameOfPeople client = submit client (nameOfPeople gIdentity) Nothing

Graph structure types

Graph structure interfaces in Gremlin are represented as type-classes in greskell. We have Element, Vertex, Edge and Property type-classes for the interfaces of the same name.

The reason why we use type-classes is that it allows you to define your own data types as a graph structure. See "Make your own graph structure types" below in detail.

Nonetheless, it is convenient to have some generic data types we can use for graph structure types. For that purpose, we have AVertex, AEdge, AVertexProperty and AProperty types.

Those types are useful because some functions are too polymorphic for the compiler to infer the types for its "start" and "end".

import Data.Greskell.Greskell (toGremlin)
import Data.Greskell.Graph (AVertex)
import Data.Greskell.GTraversal
  ( GTraversal, Transform,
    source, (&.), sV, gOut, sV', gOut',

main = hspec $ specify "monomorphic walk" $ do
  ---- This doesn't compile
  -- toGremlin (source "g" & sV [] &. gOut []) `shouldBe` "g.V().out()"

  -- This compiles, with type annotation.
  let gv :: GTraversal Transform () AVertex
      gv = source "g" & sV []
      gvo :: GTraversal Transform () AVertex
      gvo = gv &. gOut []
  toGremlin gvo `shouldBe` "g.V().out()"
  -- This compiles, with monomorphic functions.
  toGremlin (source "g" & sV' [] &. gOut' []) `shouldBe` "g.V().out()"

In the above example, sV and gOut are polymorphic with Vertex constraint, so the compiler would complain about the ambiguity. In that case, you can add explicit type annotations of AVertex type, or use monomorphic versions, sV' and gOut'.

GraphSON parser

A in AVertex stands for "Aeson". That means this type is based on the data type from Data.Aeson module. With Aeson, greskell implements parsers for GraphSON.

GraphSON is a format to encode graph structure types into JSON. As of this writing, there are three slightly different versions of GraphSON. This makes the graph structure types a little complicated.

To support GraphSON decoding, we introduced the following symbols:

  • GraphSON type: GraphSON a has data of type a and optional "type string" that describes the type of that data.
  • GValue type: basically Aeson's Value enhanced with GraphSON.
  • FromGraphSON type-class: types that can be parsed from GValue. It's analogous to Aeson's FromJSON.

AVertex, AEdge, AVertexProperty and AProperty types implement FromGraphSON instance, so they can be parsed from GraphSON v1, v2 and v3 formats.

import Data.Greskell.GraphSON
  ( nonTypedGValue, typedGValue', GValueBody(GNumber, GString)
import Data.Greskell.Graph
  ( AVertex(..), AVertexProperty(..),

vertex_GraphSONv1 = [here|
  "id" : 1,
  "label" : "person",
  "type" : "vertex",
  "properties" : {
    "name" : [ {
      "id" : 0,
      "value" : "marko"
    } ]

vertex_GraphSONv3 = [here|
  "@type" : "g:Vertex",
  "@value" : {
    "id" : {
      "@type" : "g:Int32",
      "@value" : 1
    "label" : "person",
    "properties" : {
      "name" : [ {
        "@type" : "g:VertexProperty",
        "@value" : {
          "id" : {
            "@type" : "g:Int64",
            "@value" : 0
          "value" : "marko",
          "label" : "name"
      } ]

decoded_vertex_GraphSONv1 =
  { avId = nonTypedGValue $ GNumber 1,
    avLabel = "person",
    avProperties = fromProperties [
      { avpId = nonTypedGValue $ GNumber 0,
        avpLabel = "name",
        avpValue = nonTypedGValue $ GString "marko",
        avpProperties = mempty

decoded_vertex_GraphSONv3 =
  { avId = typedGValue' "g:Int32" $ GNumber 1,
    avLabel = "person",
    avProperties = fromProperties [
      { avpId = typedGValue' "g:Int64" $ GNumber 0,
        avpLabel = "name",
        avpValue = nonTypedGValue $ GString "marko",
        avpProperties = mempty

main = hspec $ specify "GraphSON" $ do
  A.eitherDecode vertex_GraphSONv1 `shouldBe` Right decoded_vertex_GraphSONv1
  A.eitherDecode vertex_GraphSONv3 `shouldBe` Right decoded_vertex_GraphSONv3

As you can see in the above example, the vertex object in GraphSON version 3 has @type and @value fields, while version 1 does not. AVertex can parse both versions. The @type field, if present, is stored in GValue type.

Make your own graph structure types

When you use a graph database, I think you usually encode your application-specific data types as graph data structures, and store them in the database. greskell supports directly embedding your application-specific data types into graph data structures.


For example, let's make the following Person type a graph Vertex.

import Data.Greskell.Graph
  ( Element(..), Vertex, Edge(..), Property(..),
    AVertexProperty, AVertex(..), AProperty,
import Data.Greskell.GraphSON (FromGraphSON(parseGraphSON), Parser)
import Data.Greskell.Greskell (toGremlin)
import Data.Greskell.GTraversal
  ( GTraversal, Transform,
    source, sV, gHasLabel, gHas2, (&.)

data Person =
  { personId :: Int,
    personName :: Text,
    personAge :: Int

In that case, just make it instances of Element and Vertex type-classes.

instance Element Person where
  type ElementID Person = Int
  type ElementProperty Person = AVertexProperty

instance Vertex Person

Element type-class has two associated types.

  • ElementID is the type of the vertex ID. It depends on your graph database implementation and settings.
  • ElementProperty is the type of the property of the vertex. If you don't care, you can use AVertexProperty.

Once Person is a Vertex, you can use it in greskell's traversal DSL.

main = hspec $ specify "your own graph types" $ do
  let get_marko :: GTraversal Transform () Person
      get_marko = source "g" & sV [] &. gHasLabel "person" &. gHas2 "name" "marko"
  toGremlin get_marko `shouldBe` "g.V().hasLabel(\"person\").has(\"name\",\"marko\")"

In addition, you can easily implement FromGraphSON instance for Person type using AVertex.

instance FromGraphSON Person where
  parseGraphSON v = fromAVertex =<< parseGraphSON v
      fromAVertex :: AVertex -> Parser Person
      fromAVertex av = do
        guard (avLabel av == "person")
        pid <- parseGraphSON $ avId av
        name <- parseOneValue "name" $ avProperties av
        age <- parseOneValue "age" $ avProperties av
        return $ Person pid name age

Using AVertex as an intermediate type, you can now parse GraphSON (in any version!) vertex into Person type. With FromGraphSON instance, you can directly get Person from the Gremlin Server.

Like the above example of Person, you can make your own types for other graph structures.


For an Edge, make it instances of Element and Edge. You can use AProperty for ElementProperty if you don't care.

data MyEdge = MyEdge

instance Element MyEdge where
  type ElementID MyEdge = Text
  type ElementProperty MyEdge = AProperty

instance Edge MyEdge where
  type EdgeVertexID MyEdge = Integer


For a simple Property, make it instance of Property. Note that the kind of a property type has to be (* -> *).

data MyProperty v = MyProperty v

instance Property MyProperty where
  propertyKey _ = "key"
  propertyValue (MyProperty v) = v


For a VertexProperty, just make it instances of Element and Property. We don't have VertexProperty type-class, because Element and Property have different kinds. You can use AProperty for ElementProperty if you don't care.

data MyVertexProperty v = MyVertexProperty v

instance Element (MyVertexProperty v) where
  type ElementID (MyVertexProperty v) = Int
  type ElementProperty (MyVertexProperty v) = AProperty

instance Property MyVertexProperty where
  propertyKey _ = "key"
  propertyValue (MyVertexProperty v) = v


  • Complete graph traversal steps API.


Toshio Ito

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