Style

Idiomatic Yona is not just aesthetics: several of these rules change what the compiler can do for you. Flat let bindings parallelize; nested ones serialize. Each rule below shows the bad form, the good form, and why.

Never nest let

let takes multiple comma-separated bindings; nesting buries that and hurts readability.

# Bad — unnecessary nesting
let x = 42 in
let y = x + 1 in
x + y

# Good — flat multi-binding
let x = 42, y = x + 1 in x + y
# => 85

The payoff is bigger than style: independent bindings in one let run in parallel. Nested lets force sequential execution even when the bindings don’t depend on each other.

# Bad — each read waits for the previous one
let a = readFile "foo.txt" in
let b = readFile "bar.txt" in
a ++ b

# Good — both reads in flight at once; elapsed ≈ max, not sum
let a = readFile "foo.txt",
    b = readFile "bar.txt"
in a ++ b

See Concurrency for the full model.

let and do have different semantics

let binds values. Independent right-hand sides may run in parallel and are awaited at first use. do sequences effects: every step runs strictly top to bottom, even when the steps look independent. Combining them is valid — and often the right shape — when you want both:

# Good — two reads in flight, then ordered writes
let
    a = readFile "foo.txt",
    b = readFile "bar.txt"
in do
    writeFile "out-a.txt" (process a)
    writeFile "out-b.txt" (process b)
end

Putting those reads in a do would serialize them. Putting those writes in a multi-binding let would allow them to overlap. Use let when the bindings are values (and may run together); use do when order itself is the point. See Concurrency.

The anti-pattern is using let as a sequencer for an unused effect:

# Bad — discard-binding to force an effect
let _ = writeFile "out.txt" data in data

# Good — do block for side effects; last expression is the value
do
    writeFile "out.txt" data
    data
end

do also takes intermediate bindings (name = expr), executed strictly in order — the idiomatic shape for a protocol or any sequential I/O where the steps depend on each other:

do
    content = readFile "input.txt"
    result = process content
    writeFile "output.txt" result
    result
end

Do not wrap a single expression in do. Do not pad a function or program with a dummy last value such as 0 — the last real expression is the result (println already yields ()).

Comma-separate imports

Nested import expressions add a level of indentation per module for no benefit.

# Bad — one import wrapping another
import length from Std\String in
import println from Std\Io in
println (length "hello")

# Good — one import expression, comma-separated clauses
import length from Std\String, println from Std\Io in
println (length "hello")
# 5

Use with for resources

Manual close calls are lost on every early exit and exception; with releases the resource deterministically when the scope exits.

# Bad — close is skipped if send raises
do
    fd = tcpConnect "localhost" 8080
    send fd "hello"
    close fd
end

# Good — released on success or exception, checked by the Closeable trait
with fd = tcpConnect "localhost" 8080 in
    send fd "hello"

The resource type must implement Closeable — this is verified at compile time, so a with over a non-resource is an error, not a surprise at runtime.

Parallel comprehensions for concurrent work

Mapping an I/O-bound or CPU-heavy function sequentially wastes the runtime’s thread pool; [| … ] runs one task per element and keeps result order.

# Bad — one fetch at a time
[ httpGet url for url = urls ]

# Good — all fetches concurrent, results in source order
[| httpGet url for url = urls ]

[| x * 2 for x = [1, 2, 3, 4, 5] ]
# => [2, 4, 6, 8, 10]

Keep the plain form [ … ] for cheap pure bodies, where task overhead would exceed the work.

foldl for aggregation

Hand-rolled non-tail recursion over a sequence grows the call stack; Std\List.foldl is tail-recursive, which the compiler turns into a loop, so it cannot overflow.

# Bad — deep recursion, stack depth proportional to length
let sum xs = case xs of
    []    -> 0
    [h|t] -> h + sum t
end in sum bigList

# Good — foldl, constant stack
import foldl from Std\List in
foldl (\acc x -> acc + x) 0 bigList

import foldl from Std\List in
foldl (\acc x -> acc + x) 0 [1, 2, 3, 4]
# => 10

foldr exists for the cases that genuinely need right association; default to foldl.

Iterators for streaming

Reading a whole file into a sequence costs O(file) memory; iterator-based functions like readLines, chars, and split stream in O(1).

# Bad — reads the whole file into one string before counting
import readFile from Std\File, chars from Std\String, foldl from Std\List in
foldl (\n c -> if c == '\n' then n + 1 else n) 0 [c for c = chars (readFile "big.log")]

# Good — streams line by line, constant memory
import readLines from Std\File, foldl from Std\List in
foldl (\n _ -> n + 1) 0 [line for line = readLines "big.log"]

Iterator values feed comprehensions as generator sources; nothing is read from the file until the generator pulls it.

Naming conventions

Consistent casing carries information: you can tell a constructor from a function from a symbol at a glance.

# Bad
process_item x = x          # snake_case function
module std\my_utils          # lowercase module
case status of :OK -> 1 end # uppercase symbol

# Good
processItem x = x            # camelCase functions and variables
module Std\MyUtils           # PascalCase modules, backslash-separated
case status of :ok -> 1 end  # :snake_case symbols

• Functions, variables: camelCase — readFile, processItem
• Modules, types, constructors: PascalCase — Std\String, Option, Some
• Symbols: :snake_case — :ok, :not_found
• Type variables: single lowercase letters — a, b, e

Indentation

Two spaces per level, lines within 80–100 characters.

# Bad — four spaces and tab mixes drift into misalignment
case xs of
        []    -> 0
        [h|t] -> h
end

# Good — two spaces
case xs of
  []    -> 0
  [h|t] -> h
end

Newlines end expressions in case arms, do blocks, and module bodies, so consistent shallow indentation keeps expression boundaries obvious. Inside brackets, and after binary operators and ->, newlines are suppressed — use that for natural line continuation instead of escape characters.

Use the prelude

Some, None, Ok, Err, Linear, Iterator, identity, const, flip, and compose are always in scope; importing or re-defining them is noise. (Collection functions like foldl are not prelude — import them from Std\List.)

# Bad — shadowing a prelude type with a homemade one
type Maybe a = Just a | Nothing
case lookup k m of Just v -> v; Nothing -> 0 end

# Good — prelude Option, no import, no declaration
case lookup k m of
  Some v -> v
  None   -> 0
end

Prefer Result a e (Ok/Err) for fallible operations and Option a (Some/None) for absence; both pattern-match everywhere without setup.

Quick checklist

• Flat let, one binding list — independent bindings parallelize.
• let for values, do for ordered effects; combining them is fine. Never let _ = effect, never a one-line do, never a dummy trailing 0.
• One import, comma-separated clauses.
• with for anything Closeable.
• [| … ] when the body is worth a task; [ … ] otherwise.
• Std\List.foldl over hand-rolled recursion for aggregation.
• Iterators for large inputs.
• camelCase / PascalCase / :snake_case; two-space indent.
• Reach for the prelude before writing it yourself.

For the semantics behind these rules, see Concurrency, Modules, and the specification.