Servant.Scotty.Arguments

Functions and typeclasses for automatically fetching the arguments of the "database operations" from either the request path or from the request body, by decoding it from JSON to your type.

Deducing an argument from the request path

class Index k where

What it means for a scotty Resource to have an index type.

• idx should lookup in the request path whatever is necessary to get the i of Resource c a i r e ops, for operations that take it as an argument, e.g Delete, Update or View.
• route should return a String that'll be passed to capture. You may use one or more "path parameters" (calls to param, instances of Param) to compute your value of type k. You probably want to use name on the Resource to generate the beginning of the path.

Methods

idx :: (Functor m, MonadIO m, ScottyError e) => ActionT e m k

Lookup the index in the request path

route :: Resource c a k r e ops -> String

String to capture that represents the RoutePattern.

Deducing an argument from the JSON body

js :: (MonadIO m, ScottyError e, FromJSON a) => ActionT e m a

Simply gets the request's body as JSON (or raises an exception if the decoding fails)

class FromJSON a

A type that can be converted from JSON, with the possibility of failure.

When writing an instance, use empty, mzero, or fail to make a conversion fail, e.g. if an Object is missing a required key, or the value is of the wrong type.

An example type and instance:

{-# LANGUAGE OverloadedStrings #-}

data Coord = Coord { x :: Double, y :: Double }

instance FromJSON Coord where
   parseJSON (Object v) = Coord    <$>
                          v .: "x" <*>
                          v .: "y"

-- A non-Object value is of the wrong type, so use mzero to fail.
   parseJSON _          = mzero

Note the use of the OverloadedStrings language extension which enables Text values to be written as string literals.

Instead of manually writing your FromJSON instance, there are three options to do it automatically:

• Data.Aeson.TH provides template-haskell functions which will derive an instance at compile-time. The generated instance is optimized for your type so will probably be more efficient than the following two options:
• Data.Aeson.Generic provides a generic fromJSON function that parses to any type which is an instance of Data.
• If your compiler has support for the DeriveGeneric and DefaultSignatures language extensions, parseJSON will have a default generic implementation.

To use this, simply add a deriving Generic clause to your datatype and declare a FromJSON instance for your datatype without giving a definition for parseJSON.

For example the previous example can be simplified to just:

{-# LANGUAGE DeriveGeneric #-}

import GHC.Generics

data Coord = Coord { x :: Double, y :: Double } deriving Generic

instance FromJSON Coord

Note that, instead of using DefaultSignatures, it's also possible to parameterize the generic decoding using genericParseJSON applied to your encoding/decoding Options:

 instance FromJSON Coord where
     parseJSON = genericParseJSON defaultOptions

Instances

FromJSON Bool

FromJSON Char

FromJSON Double

FromJSON Float

FromJSON Int

FromJSON Int8

FromJSON Int16

FromJSON Int32

FromJSON Int64

FromJSON Integer

WARNING: Only parse Integers from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like 1e1000000000.

FromJSON Word

FromJSON Word8

FromJSON Word16

FromJSON Word32

FromJSON Word64

FromJSON ()

FromJSON Scientific

FromJSON Number

FromJSON Text

FromJSON UTCTime

FromJSON Value

FromJSON DotNetTime

FromJSON Text

FromJSON IntSet

FromJSON ZonedTime

FromJSON [Char]

FromJSON a => FromJSON [a]

FromJSON (Ratio Integer)

FromJSON a => FromJSON (Maybe a)

HasResolution a => FromJSON (Fixed a)

WARNING: Only parse fixed-precision numbers from trusted input since an attacker could easily fill up the memory of the target system by specifying a scientific number with a big exponent like 1e1000000000.

FromJSON a => FromJSON (Dual a)

FromJSON a => FromJSON (First a)

FromJSON a => FromJSON (Last a)

FromJSON v => FromJSON (Tree v)

FromJSON a => FromJSON (IntMap a)

(Ord a, FromJSON a) => FromJSON (Set a)

(Eq a, Hashable a, FromJSON a) => FromJSON (HashSet a)

FromJSON a => FromJSON (Vector a)

(Vector Vector a, FromJSON a) => FromJSON (Vector a)

(Storable a, FromJSON a) => FromJSON (Vector a)

(Prim a, FromJSON a) => FromJSON (Vector a)

(FromJSON a, FromJSON b) => FromJSON (Either a b)

(FromJSON a, FromJSON b) => FromJSON (a, b)

FromJSON v => FromJSON (HashMap String v)

FromJSON v => FromJSON (HashMap Text v)

FromJSON v => FromJSON (HashMap Text v)

FromJSON v => FromJSON (Map String v)

FromJSON v => FromJSON (Map Text v)

FromJSON v => FromJSON (Map Text v)

(FromJSON a, FromJSON b, FromJSON c) => FromJSON (a, b, c)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d) => FromJSON (a, b, c, d)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e) => FromJSON (a, b, c, d, e)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f) => FromJSON (a, b, c, d, e, f)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g) => FromJSON (a, b, c, d, e, f, g)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h) => FromJSON (a, b, c, d, e, f, g, h)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i) => FromJSON (a, b, c, d, e, f, g, h, i)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j) => FromJSON (a, b, c, d, e, f, g, h, i, j)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k) => FromJSON (a, b, c, d, e, f, g, h, i, j, k)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n)

(FromJSON a, FromJSON b, FromJSON c, FromJSON d, FromJSON e, FromJSON f, FromJSON g, FromJSON h, FromJSON i, FromJSON j, FromJSON k, FromJSON l, FromJSON m, FromJSON n, FromJSON o) => FromJSON (a, b, c, d, e, f, g, h, i, j, k, l, m, n, o)