# Functions

Functions have two types of parameters:

1. Positional parameters, which require an argument.
2. Named parameters, which optionally take an argument, otherwise using their default value.

So this function is named fahrenheit_to_celsius and has one parameter temp:

#### PRQL

let fahrenheit_to_celsius = temp -> (temp - 32) / 1.8

from cities
derive temp_c = (fahrenheit_to_celsius temp_f)

#### SQL

SELECT
*,
(temp_f - 32) / 1.8 AS temp_c
FROM
cities

This function is named interp, and has two positional parameters named high and x, and one named parameter named low which takes a default argument of 0. It calculates the proportion of the distance that x is between low and high.

#### PRQL

let interp = low:0 high x -> (x - low) / (high - low)

from students
derive {
sat_proportion_1 = (interp 1600 sat_score),
sat_proportion_2 = (interp low:0 1600 sat_score),
}

#### SQL

SELECT
*,
(sat_score - 0) / (1600 - 0) AS sat_proportion_1,
(sat_score - 0) / (1600 - 0) AS sat_proportion_2
FROM
students

## Other examples

#### PRQL

let is_adult = col -> col >= 18
let writes_code = col -> (col | in ["PRQL", "Rust"])
let square = col -> (col | math.pow 2)
let starts_with_a = col -> (col | text.lower | text.starts_with("a"))

from employees
select {
first_name,
last_name,
hobby,
age_squared = square age,
}
filter ((starts_with_a last_name) && (writes_code hobby))

#### SQL

WITH table_0 AS (
SELECT
first_name,
last_name,
hobby,
POW(age, 2) AS age_squared
FROM
employees
)
SELECT
first_name,
last_name,
hobby,
age_squared
FROM
table_0
WHERE
LOWER(last_name) LIKE CONCAT('a', '%')
AND hobby IN ('PRQL', 'Rust')

## Piping values into functions

Consistent with the principles of PRQL, it’s possible to pipe values into functions, which makes composing many functions more readable. When piping a value into a function, the value is passed as an argument to the final positional parameter of the function. Here’s the same result as the examples above with an alternative construction:

#### PRQL

let interp = low:0 high x -> (x - low) / (high - low)

from students
derive {
sat_proportion_1 = (sat_score | interp 1600),
sat_proportion_2 = (sat_score | interp low:0 1600),
}

#### SQL

SELECT
*,
(sat_score - 0) / (1600 - 0) AS sat_proportion_1,
(sat_score - 0) / (1600 - 0) AS sat_proportion_2
FROM
students

and

#### PRQL

let fahrenheit_to_celsius = temp -> (temp - 32) / 1.8

from cities
derive temp_c = (temp_f | fahrenheit_to_celsius)

#### SQL

SELECT
*,
(temp_f - 32) / 1.8 AS temp_c
FROM
cities

We can combine a chain of functions, which makes logic more readable:

#### PRQL

let fahrenheit_to_celsius = temp -> (temp - 32) / 1.8
let interp = low:0 high x -> (x - low) / (high - low)

from kettles
derive boiling_proportion = (temp_c | fahrenheit_to_celsius | interp 100)

#### SQL

SELECT
*,
((temp_c - 32) / 1.8 - 0) / (100 - 0) AS boiling_proportion
FROM
kettles

### Late binding

Functions can bind to any variable that is in scope when the function is executed. For example, here cost_total refers to the column that’s introduced in the from.

#### PRQL

let cost_share = cost -> cost / cost_total

from costs
derive {
materials_share = (cost_share materials),
labor_share = (cost_share labor),
}

SELECT
materials,
labor,