01. Basic data and immutable collections

Hara programs start with values. Most values are immutable. An operation returns a new value instead of changing the old value.

This lesson introduces scalar values, collection literals, lookup, and persistent updates.

Learning goals

By the end of this lesson, you can:

1. Read the main Hara literal forms.
2. Distinguish a list form from a vector value.
3. Use keywords to label data.
4. Read values from vectors and maps.
5. Update a persistent collection without mutation.
6. Choose a collection from the job it performs.

Scalar values

A scalar value is one value that is not a collection.

Nil and booleans

Hara has one empty value and two boolean values:

nil
true
false

Only nil and false are falsey. Zero, an empty string, and an empty collection are truthy.

(if 0 :yes :no)
; => :yes

(if [] :yes :no)
; => :yes

Do not use emptiness as a boolean test. Use an explicit predicate such as empty?.

Numbers

Hara supports integers and floating-point values:

42
-7
3.5

It also reads big integers and big decimals:

12345678901234567890N
19.95M

The suffix is part of the numeric literal.

Ratios are not an L0 numeric type. Division does not create a ratio value.

(/ 5 2)

Check the result in your current runtime before you build numeric rules around integer division.

Strings and characters

A string contains text:

"HAL"
"line one\nline two"

A character is a single character value:

\a
\newline

Strings are immutable. String transformation functions return new strings.

Keywords

A keyword is a stable label:

:name
:task/title
:status/ready

Keywords often label fields in maps. A namespaced keyword makes the domain explicit.

{:task/title "Read input"
 :task/status :status/ready}

Symbols

A symbol names a Var, local binding, function, namespace, or other program entity:

score
map
file/read

The evaluator resolves an unquoted symbol. Quote returns the symbol itself:

'file/read
; => file/read

A keyword usually labels data. A symbol usually names program behavior or a binding.

Collection literals

Hara has lists, vectors, maps, and sets.

Lists

A list uses parentheses:

(+ 19 23)

In source code, the evaluator normally treats a list as a call or special form.

Quote a list when you want list data:

'(north east south west)

Use lists for forms and list-shaped data. Do not use a list only because HAL uses parentheses for evaluation forms.

Vectors

A vector uses square brackets:

["alpha" "beta" "gamma"]

A vector preserves order and supports indexed access:

(nth [10 20 30] 1)
; => 20

Indexes start at zero.

Use a vector for ordered records, argument lists, coordinates, and finite sequences that need indexed access.

Maps

A map associates keys with values:

{:task/id 1
 :task/title "Read input"
 :task/done false}

Use get to read a key:

(get {:task/title "Read input"} :task/title)
; => "Read input"

A keyword can also look itself up in a map:

(:task/title {:task/title "Read input"})
; => "Read input"

Use a map when fields have names.

Sets

A set contains unique values:

#{:read :transform :write}

Adding an existing value does not create a duplicate:

(conj #{:read :write} :read)
; => #{:read :write}

Use a set for membership, permissions, tags, and unique categories.

Persistent updates

A persistent collection does not change in place. An update returns another collection.

Add a vector item

(def steps [:read :transform])
(def next-steps (conj steps :write))

steps
; => [:read :transform]

next-steps
; => [:read :transform :write]

The old vector remains valid.

Replace a map field

(def task
  {:task/id 1
   :task/title "Read input"
   :task/done false})

(def completed-task
  (assoc task :task/done true))

assoc returns a map with the selected key associated with the new value.

Remove a map field

(dissoc task :task/title)
; => {:task/id 1 :task/done false}

The original task value still contains the title.

Update nested data

Use get-in and assoc-in for nested paths:

(def job
  {:job/id 7
   :job/progress {:current 0 :total 3}})

(get-in job [:job/progress :total])
; => 3

(assoc-in job [:job/progress :current] 1)

Use update-in when the new value depends on the old value:

(update-in job [:job/progress :current] inc)

Each operation returns a new root map.

Collection operations preserve the boundary

Persistent operations return persistent values:

(conj [1 2] 3)
(assoc {:a 1} :b 2)
(dissoc {:a 1 :b 2} :a)

These operations do not silently create mutable arrays or objects.

Later, you will use array and object when mutation is intentional.

Build the course configuration

Create one immutable configuration value:

(def config
  {:input/path "data/input.txt"
   :output/path "data/output.txt"
   :pipeline/steps [:trim :remove-empty :number]
   :pipeline/tags #{:text :utf8}
   :pipeline/options
   {:skip-empty true
    :max-lines 1000}})

Read one value from each collection level:

(:input/path config)

(nth (:pipeline/steps config) 1)

(get-in config [:pipeline/options :max-lines])

Create a changed configuration without changing config:

(def small-config
  (assoc-in config [:pipeline/options :max-lines] 10))

Practice loop

Use this sequence for each form:

1. Predict the result.
2. Run the form.
3. Change one literal.
4. Explain which value changed.

Try these changes:

1. Add another pipeline step.
2. Add a tag to the set.
3. Change the maximum line count.
4. Remove the output path.
5. Confirm that the original value remains unchanged.

Common mistakes

Treating parentheses as a container

(1 2 3)

The evaluator tries to call 1. Quote list data or use a vector.

Expecting assoc to mutate

(assoc config :input/path "other.txt")
config

The second form returns the original value because the first result was not stored or passed onward.

Using truthiness for emptiness

(if [] :empty :not-empty)
; => :empty

This label is misleading. Empty collections are truthy. Use empty?.

Mixing field names

Do not call the same field :path, :file, and :input in nearby examples. Choose one stable key such as :input/path.

Check yourself

You are ready for the next lesson when you can answer these questions:

1. Which Hara values are falsey?
2. Why does a list often represent code?
3. When should you use a vector instead of a map?
4. What does assoc return?
5. Why can old and new persistent values coexist?
6. When is a set a better fit than a vector?
7. What is the difference between a keyword and a symbol?

Continue with 02. Functions and atoms.