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,
    adult = is_adult age,
    age_squared = square age,
}
filter ((starts_with_a last_name) && (writes_code hobby))

SQL

WITH table_0 AS (
  SELECT
    first_name,
    last_name,
    hobby,
    age >= 18 AS adult,
    POW(age, 2) AS age_squared
  FROM
    employees
)
SELECT
  first_name,
  last_name,
  hobby,
  adult,
  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
select {materials, labor, overhead, cost_total}
derive {
  materials_share = (cost_share materials),
  labor_share = (cost_share labor),
  overhead_share = (cost_share overhead),
}

SQL

SELECT
  materials,
  labor,
  overhead,
  cost_total,
  materials / cost_total AS materials_share,
  labor / cost_total AS labor_share,
  overhead / cost_total AS overhead_share
FROM
  costs