CODEWITHSUNDEEP
javascriptfunctional-programmingramda.js
Himmel
Himmel

Reputation: 3709

Ramda `cond` else statement

I'm looking at the Ramda docs for the cond function and am confused about its behavior. The docs state that cond...

Returns a function, fn, which encapsulates if/else-if/else logic. R.cond takes a list of [predicate, transform] pairs. All of the arguments to fn are applied to each of the predicates in turn until one returns a "truthy" value, at which point fn returns the result of applying its arguments to the corresponding transformer. If none of the predicates matches, fn returns undefined.

Here is the example given:

var fn = R.cond([
  [R.equals(0),   R.always('water freezes at 0°C')],
  [R.equals(100), R.always('water boils at 100°C')],
  [R.T,           temp => 'nothing special happens at ' + temp + '°C']
]);

fn(0); //=> 'water freezes at 0°C'
fn(50); //=> 'nothing special happens at 50°C'
fn(100); //=> 'water boils at 100°C'

I understand the [predicate, transform] aspect of the function, but it isn't clear to me how the "else" portion works. In a typical if/else-if/else statement, the "else" portion does not accept a predicate. In the example, however, each of the arrays have a predicate. Maybe knowing how R.T operates in this case would help, but searching for T in the docs was fruitless.

How can I use Ramda's cond function to capture conditional "else" functionality in order to return a default value?

Upvotes: 2

Views: 4502

Answers (1)

Karpak
Karpak

Reputation: 1927

R.T always returns true and ignores any parameters passed to it. That is the reason the 100 you passed in was ignored and it just returned true.

R.cond searches each of the [predicate, transform] pairs, and stops searching on the first predicate that evaluates to true. So the first matching entity from the [predicate, transform] pair would be evaluated.

If nothing is true, then it reaches the end and executes the R.T predicate (which is always true) and that acts like the else part of list.

Upvotes: 5

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