Ramda: ComposeK is not working

[MengWeiChen]

Hi,

I am trying to use composeK but I got some error.

Here is my code

const studentList = [{
  name: 'dOg',
  nickname: ' goofY  ',
  score: {
    js: 7,
    html: 8,
    css: 9,
  },
}, {
  name: '  caT  ',
  nickname: 'tom',
  score: {
    js: 6,
    html: 7,
    css: 8,
  },
}, {
  name: 'MOUSE',
  nickname: '  jerry',
  score: {
    js: 8,
    html: 9,
    css: 10,
  },
}];

const getScore = R.prop('score');

const getJs = R.prop('js');

const getJsScore = R.composeK(getJs, getScore);

getJsScore(studentList); // -> [7, 6, 8]

Online link is here

When I tried to run those code, I got an error message Cannot read property 'fantasy-land/chain' of undefined

I don't know is my code have any problem or composeK is broken.

Can any give me some help?

[Bradcomp]

Hi @MengWeiChen,

We can take a look at the type signatures in play here to see what is going on with your code. An explanation of the type signatures is available here. composeK has a fairly restrictive type signature, so it isn't used all that often in my experience.

composeK :: Chain m => ((b -> m c), (a -> m b)) -> (a -> m c)

studentList :: Array Object

getScore :: Object -> Object

getJs :: Object -> Number

Because Array sarisfies the type constraint, it can be used as the m in Chain m, however, none of the functions you're composing _return_ a Chain which is the requirement for using composeK. In fact, in this case, you'll want to use map to appy a function to a list of values.

map :: Functor f :: (a -> b) -> f a -> f b

Once we replace Functor f with Array we get (a -> b) -> Array a -> Array b.

All type signatures aside, if you are looking at running a function over a list of values, you're almost always going to want to use map.

We can then use a regular compose for our two prop calls:

const getJsScore = compose(getJs, getScore);

So the final function could be used like this:

map(getJsScore, studentList) Link

Lastly, Ramda has a function called path which will simplify your mapping function. Instead of compose(getJs, getScore) you can just do path(['score', 'js']) so the final solution would be:

map(path(['score', 'js']), studentList) //[7, 6, 8].

Please let me know if that's helpful at all. In addition to asking questions here, we also have a chat room that's very active, and you'll find a lot of helpful people in there.