Const and Let Bindings

Infr introduces const and let keywords to control variable reassignment — something R has never had.

const — prevent reassignment

const prevents a binding from being reassigned:

const pi_approx <- 3.14159
pi_approx <- 2.0
# Error [infr]: Cannot reassign const binding `pi_approx` (declared at line 1)

const name <- "Alice"
name <- "Bob"
# Error [infr]: Cannot reassign const binding `name` (declared at line 4)

Const is shallow

const prevents rebinding the name, not deep mutation of the underlying object. This matches R's copy-on-modify semantics:

const df <- data.frame(x = 1:3, y = 4:6)

df$z <- 7:9
# Warning [infr]: Mutating const binding `df` via `$<-`.
# This creates a modified copy. Consider using `let` if mutation is intended.

df <- other_df
# Error [infr]: Cannot reassign const binding `df` (declared at line 1)

let — explicit mutability

let declares a binding that you intend to reassign:

let counter <- 0
counter <- counter + 1    # OK

let accumulator <- c()
accumulator <- c(accumulator, new_value)  # OK

Plain <- — standard R

The regular <- operator still works exactly as in R. It creates a mutable binding:

x <- 10
x <- 20  # OK — standard R behavior

When to use which?

Keyword	Use when...
const	The value should never change (config, constants, final results)
let	You intend to reassign (counters, accumulators, loop variables)
<-	Writing standard R code, or not ready to commit to const/let

Strictness levels

How Infr treats bare <- depends on the strictness level:

Level	Bare <- behavior
relaxed (default)	Treated as mutable, no warnings
moderate	Treated as mutable, warns "consider const or let"
strict	Error: must use const or let