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

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