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 always scalars. They cannot be ranvar, zedfunc, or embedding, and tuples are not supported.

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}!"

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.