const

const, keyword, assignment option

The const keyword indicates that the value taken by a variable is known at compile time and that it won’t change later at runtime. Those variables are primarily intended to control the files being read.

const myFolder = "/foo"

read "\{myFolder}/products.csv" as Products with // pattern in 'read' must be constant
  Product: text
  Price: number

Notably, the read block requires a text value for its file path that evaluates as const.

Unlike many other languages, in Envision, const is a somewhat rarely used keyword, mostly used for certain capabilities that do not accept anything but compile-time constants (ex: read blocks).

Where const is required

Compile-time constants are required for:

• read patterns and bounded path schema constraints.
• loop iteration counts.
• match patterns (patterns cannot contain variables).
• module exports flagged as const.
• expressions inside def const pure functions.
• context capture values used by user-defined functions.

Constraints

Compile-time constants are scalars or tuples of scalars. Every tuple element must be a compile-time constant. They cannot contain ranvar, zedfunc, or embedding values.

Allowed constant expressions

Arithmetic and math operations:

const a = 10 + 25

Enum conversions:

table enum Currency = "EUR", "USD"
const EnumUSD = enum<<Currency>>("USD")
const TextUSD = text(EnumUSD)

Const pure functions:

const four = strlen("four")

Conditional expressions:

const label = if 1 < 2 then "yes" else "no"

Text interpolation:

const hello = "Hello"
const world = "World"
const greet = "\{hello}, \{world}!"

Tuples and unpacking:

const config = ("EUR", 2)
const currency, digits = config

show summary "Settings" a1d2 with
  currency
  digits

The tile shows “EUR” and 2. Tuples of constants can also be returned by def const pure functions or exported from modules.

The constant() function is unrelated to const and does not create compile-time constants.

const, keyword, pure function option

The const keyword indicates that the pure function can be executed at compile time.

def const pure myInc(a: number) with 
  return a + 1

const a = 42
// if 'myInc' is called with a 'const' input, then, its output is 'const' too
const x = myInc(a)

const myFolder = "/foo"

read "\{myFolder}/products-\{x}.csv" as Products with // pattern in 'read' must be constant
  Product: text
  Price: number

In the present reference documentation, pure functions that are part of the Envision standard library and that can be executed at compile time are marked as const.