2. Data types

Every value in HQL has a type, and the checker knows it before the program runs. This chapter meets the types values are built from: the scalars Int, Float, Decimal, String and Bool, and Data, the tree JSON and YAML are made of. Each has a page of its own; this one tries them in the REPL first.

Trying values in the REPL

hql repl reads one input at a time, evaluates it and prints the value. A binding is kept for the inputs after it, and :type asks for a type without evaluating anything:

$ hql repl
hql 0.5.0. `:help` for commands, `:quit` to leave.
hql> pages = 412
()
hql> pages / 3
137
hql> pages % 3
1
hql> :type pages
Int
hql> title = "Dune"
()
hql> title.upper()
"DUNE"
hql> :type title
String
hql> fmt("{} has {} pages", title, pages)
"Dune has 412 pages"
hql> :quit

412 is an Int, so / divides whole numbers and drops the remainder, and % gives the remainder. "Dune" is a String, and title.upper() calls a function of String on it. fmt writes values into text, one {} each.

The same programs run with hql eval, as every program in this book does:

pages = 412
title = "Dune"
fmt("{} has {} pages", title, pages)

"Dune has 412 pages"

A type is checked, not guessed

Each operation says which types it takes. An Int and a String do not add, and the program is refused before it runs:

412 + "pages"

type error: addition requires two Int, two Float or two Decimal operands
 --> <expression>:1:7
  |
1 | 412 + "pages"
  |       ^^^^^^^

There is no implicit conversion between types, except that an Int beside a Decimal widens to one. str writes a value as text, and float turns an Int into a Float:

412.str()

"412"

hql check prints a program's type without evaluating it:

$ hql check - <<< '412 / 3'
Int

The types, one page each

Type	What it holds	Page
Int	a whole number, 64 bits	Int
Float	a binary floating-point number, written 2.5f	Float
Decimal	an exact decimal number, written 2.50	Decimal
String	text	String
Bool	true or false	Bool
Data	a tree of records, lists and scalars	Data
Option<T>	a T, or nothing	Option

Braces write three different things — a set, a Data record and, where one is declared, a map — and sets, Data and maps tells them apart. Lists, sets and maps as collections, and the steps they share, are chapter 3.