CODEWITHSUNDEEP
jsonhaskellhashmap
altern
altern

Reputation: 5949

Not in scope: `HashMap.lookup'

I am having trouble with HashMap.lookup function. Please give your advise on how to fix following code (I am getting Not in scope: `HashMap.lookup' error):

instance JSON.FromJSON Document where
  parseJSON (JSON.Object v) = maybe mzero parser $ HashMap.lookup "document" v
    where parser (JSON.Object v') = Document <$> v' JSON..: "name"
                                        <*> v' JSON..: "content"
          parser _           = mzero
  parseJSON _          = mzero

Upvotes: 1

Views: 220

Answers (2)

Bakuriu
Bakuriu

Reputation: 102009

It depends on how you are importing the package. If you are using something like:

import Data.HashMap

Then you can access the Data.HashMap.lookup function as simply:

lookup

however you may have a conflict with Prelude.lookup, in this case you can either add an explicit Prelude import hiding the lookup function, i.e. add:

import Prelude hiding (lookup)

at the top of the file, or use the full name:

Data.HashMap.lookup

Note that you cannot simply use HashMap.lookup. The module is called Data.HashMap not just HashMap.

You may want to give an alias to the package, such as:

import Data.HashMap as H

And then use:

H.lookup key hashmap

This will still allow to use non-prefixed version of types/functions if they don't overlap with something else. If you want to be safe you can use a qualified import:

import qualified Data.HashMap as H

In this way everything from the HashMap module must be prefixed with H.. Even the types:

-- note: H.HashMap *not* HashMap:
somefunction :: H.HashMap String [Int] -> [Int]

Without qualified you could still write:

somefunction :: HashMap String [Int] -> [Int]

And you'd have to use H. only when calling some functions.

Note that there are different packages proving HashMap. You may want to use Data.HashMap.Lazy or Data.HashMap.Strict.

Upvotes: 3

bmk
bmk

Reputation: 1568

Try to add

import qualified Data.HashMap.Strict as HashMap

on top of your source file.

Upvotes: 3

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