Values, Variables, and Basic Types

This chapter covers numbers, strings, and mutable state. Each example runs as a single file: save it and execute tauto run filename.

3.1 Give values names

let names a value. var creates a cell whose contents can be changed. Assignment uses :=; new bindings also use let x := value and var x := value.

ch03-let.taut

let a := 10
let b: i64 := 32
perform show("sum", a + b)

var count: i64 := 0
count := count + 1
perform show("count", count)

// A mutable binding can change, while a let binding cannot be reassigned.

Execution result

sum = 42
count = 1

sum = 42
count = 1

A let binding cannot be reassigned; a var binding can. Ownership rules apply to both (Chapter 7).

3.2 Types determine what numbers mean

Numeric types have explicit meanings.

Type	Meaning	Key point
`i32`, `i64`	Fixed-width signed integers	Arithmetic overflow is a runtime error
`u8`, `u32`, `u64`	Fixed-width unsigned integers	Arithmetic does not implicitly convert widths
`Decimal`	Exact decimal numbers	Useful for decimal calculations such as money
`Float`	Floating-point numbers	Account for approximation error
`Bool`	`true` / `false`	Used in conditions
`Text`	Strings	Shared data that can be duplicated
`()`	The unit type	Computations with no meaningful result
`Never`	A type with no values	Used for computations that do not return

Tauto does not automatically mix i64 and i32 in addition. Trying it produces a type error.

ch03-mix-error.taut

let a: i32 := 1
let b: i64 := 2
perform show("x", a + b)

// This deliberately mixes integer widths and must be rejected.

✗ Error

✗ program

error 3:19: `+` of i32 and i64: the operands differ

ch03-mix-error.taut:3:19: type error: `+` of i32 and i64: the operands differ

To change widths, use explicit conversions such as I64::of_i32 or I32::of_i64. A value outside the destination's range is not silently rounded; it causes a runtime error (the number is too large).

ch03-convert.taut

let a: i32 := 1
let b: i64 := 2
perform show("sum", I64::of_i32(a) + b)
perform show("back", I32::of_i64(b))

// Convert the integer width explicitly before adding.

Execution result

sum = 3
back = 2

sum = 3
back = 2

3.3 Arithmetic and comparisons

Integer division does not use /. Use the dedicated functions div(a, b) and mod(a, b).

ch03-arith.taut

let a := 7
let b := 3
let product := a * b
let quotient := div(a, b)
let remainder := mod(a, b)
let same := a == b
perform show("p", product)
perform show("q", quotient)
perform show("r", remainder)
perform show("same", same)

// Observe multiplication, division, remainder, and comparison separately.

Execution result

p = 21
q = 2
r = 1
same = false

p = 21
q = 2
r = 1
same = false

Writing 7 / 2 produces an error explaining the available forms:

type error: integer `/` is a type error: write div(a, b), mod(a, b), or decimal(a) / decimal(b)

Division by zero is a runtime error. It is not necessarily ruled out before execution.

3.4 Strings

Strings have type Text. Double-quoted literals ("...") process escapes, while single-quoted literals ('...') do not.

ch03-text.taut

import std::prelude::Shared

let greeting := "Hello, Tauto!"
let path := 'C:\Users\example\Documents'
let joined := concat(greeting, " Good day")
perform show("text", joined)
perform show("path", path)

// Concatenate text explicitly and preserve backslashes in the raw path.

Execution result

text = "Hello, Tauto! Good day"
path = "C:\\Users\\example\\Documents"

text = "Hello, Tauto! Good day"
path = "C:\\Users\\example\\Documents"

show formats values for display, so the path's backslashes appear doubled in the output. The value itself is C:\Users\example\Documents.

To concatenate Text, use concat, not +. The expression "x" + "y" is a type error: +` of Text: not a number. Numeric and string operations are designed to have an unambiguous meaning.

Exercise 3.1: Bind two integers with let and observe their sum, difference, and product with show.