# Fenl FAQ

# Choosing between `when`

and `if`

`when`

and `if`

Fenl provides two predication functions:

`if(lhs, rhs)`

returns the`lhs`

value if the value of`rhs`

is true and returns`null`

in all other cases. The function's result follows the standard continuity rules, for example the result will be continuous if`lhs`

and`rhs`

are both continuous, otherwise the result will be discrete.`when(lhs, rhs)`

returns the value of`lhs`

every time`rhs`

produces the value`true`

. The result of`when`

is*always*discrete, and produces values at the set of times`rhs`

produces the value`true`

.

These functions filter values in different ways: `if`

filters by replacing values with `null`

, while `when`

filters values by limiting the set of times at which values are produced. Kaskada's compute engine is tabular, so in some cases the performance of `if`

will be better than that of `when`

because `if`

can be applied as a simple transformation while `when`

requires building a new table. In other cases `when`

may be performant if it allows subsequent operations to be computed over a table with significantly fewer rows.

A general guideline is to use `if`

for replacing values, and `when`

for filtering values.

For example, use `if`

and `else`

to clean values:

```
Event.duration | if($input > 0) | else(0)
```

Alternately, use `when`

to filter rows returned by a query:

```
Event | when(Event.kind == "conversion")
```

# Joining expressions with different entities

The values produced by all Fenl expressions are either constant or associated with an entity. For example the expression `42`

produces a constant integer value. By comparison the expression `ProductReview.stars`

might produce integer values associated with each review's entity, for example a particular product.

Aggregations (ie `sum`

, `min`

, `first`

) are scoped to each value's entity key. Simple functions that accept multiple arguments (ie `add`

or `eq`

) require that their non-constant arguments have compatible entity key types, and operate between values with the same entity key.

This scoping behavior makes it easy to write common operations, but often times it's necessary to combine values with different entity keys. The `lookup`

function support supports these use cases. The lookup function takes two arguments: the first argument (the key expression) describes the entity key being looked up, and the second argument (the foreign expression) describes the value to be looked up:

```
lookup(key, foreign)
```

The key expression identifies the "other" entity key, and should be expressed as an operation on "this" entity's values. For example if we want to lookup some information about the reviewer associated with each individual product review, the key expression might be something like `ProductReview.reviewer_id`

.

The foreign expression describes the value to look up, and should be expressed as an operation on the "other" entity's values. Continuing the prior example, if we wanted to know the average number of stars a reviewer gives products, the foreign expression might be something like `ProductReviewByReviewer.stars | mean()`

.

Using these two expressions it's possible to describe some facts about each product review:

```
{
product_id: ProductReview.product_id,
reviewer: ProductReview.reviewer_id,
reviewer_avg: lookup(ProductReview.reviewer_id, ProductReviewByReviewer.stars | mean()),
reviewer_count: lookup(ProductReview.reviewer_id, ProductReviewByReviewer | count()),
}
```

A lookup expression produces the value of the foreign expression at every time the key expression produces a non-null value.

Updated 4 months ago