1. Hello world¶

HQL is a statically typed expression and query language. A program is an expression, or a few statements ending in one; hql checks its type, then evaluates it, and writes the value. This chapter is about that loop.

Hello, world¶

The smallest program that says hello is the text itself:

"Hello, world!"

"Hello, world!"

A program is an expression, and its value is what hql writes. fmt writes values into a template, one {} each:

fmt("Hello, {}!", "world")

"Hello, world!"

Running a program¶

hql eval takes a program as an argument:

hql eval '"Hello, world!"'

Every program in this book runs that way. Most of them run against the tutorial vault, a small reading club in examples/tutorial/, so the commands in later chapters start with hql --vault examples/tutorial eval. The book shows each program in an hql block and what hql writes in the block after it.

1 + 2

Values and their types¶

Every value has a type: 1 + 2 is an Int, "Hello" a String. hql check writes a program's type without evaluating it:

$ hql check - <<< '7 / 2'
Int

Chapter 2 meets the types one by one.

Checked before it runs¶

A program is checked as a whole before anything is evaluated. A type error names the expression it found and points at it:

1 + "two"

type error: addition requires two Int, two Float or two Decimal operands
 --> <expression>:1:5
  |
1 | 1 + "two"
  |     ^^^^^

Nothing ran: there is no value, only the report, and the exit status is 1. The same holds for a name nobody bound:

pages + 1

name error: `pages` is not bound
 --> <expression>:1:1
  |
1 | pages + 1
  | ^^^^^

Bindings¶

name = expression binds a value to a name for the statements after it. The program's value is its last expression.

pages = 280
chapters = 24
pages / chapters

A binding may state its type, and the checker holds the value to it:

rating : Int = 5
rating

Choosing a value¶

a given condition otherwise b is a when the condition holds, and b otherwise. It is an expression, so it has a type: both branches have one.

pages = 412
"long" given pages > 300 otherwise "short"

"long"

match compares one value against patterns, in order, and the first that fits gives the result:

status = "reading"
match status {
    "done" => "finished",
    "reading" => "on the nightstand",
    _ => "on the shelf",
}

"on the nightstand"

A match that does not cover every case is refused before it runs; _ is the pattern that fits anything.

What is next¶

Chapter 2 meets the data types: Int, Float, String and Bool, tried in the REPL, and Data, the records and lists that JSON and YAML are made of.