`when()`

Function in Authoring

Dispatch a value on a condition, returning the value instead of mutating a var.

import { when } from 'typeshade'

The signature, the description and the examples come from the compiler's own source at commit 26de7be8.

Syntax

function when<K extends string>(
  cond: ReadonlyNode<'bool'>,
  thenVal: () => ReadonlyNode<K>,
  elseVal: () => ReadonlyNode<K>,
): Node<K>;
function when<K extends string>(
  arms: ReadonlyArray<readonly [ReadonlyNode<'bool'>, () => ReadonlyNode<K>]>,
  elseVal: () => ReadonlyNode<K>,
): Node<K>;

Parameters

cond ReadonlyNode<'bool'>: the test, for the two-arm shape.
thenVal () => ReadonlyNode<K>: the value when cond holds.
elseVal () => ReadonlyNode<K>: the value when no condition holds.
arms: condition and value pairs, for the N-arm shape, evaluated in order.

Return value

Node<K>

a node reading the chosen value.

Description

Two shapes are accepted: when(cond, () => a, () => b) for two arms, and when([[c0, () => e0], [c1, () => e1]], () => eN) for N arms, where the first true condition wins.

Each arm returns its value; when materialises the var and the if/elif/else chain internally, so the emit is identical to the hand-written var v; if (c) v = … form. The arms take values only, with no var name and no type token: the result type comes from the arms.

Choosing between the three dispatch surfaces is a question about the subject. when is for condition and range dispatch, where each arm tests something different and there is no single subject: a threshold ladder, a pair of unrelated flags. Switch and matchExpr are for dispatch on one integer scrutinee, the value that decides which arm runs; matchEnum is the same with an exhaustiveness check. select is the eager two-way form, which evaluates both arms.

ifExpr and condExpr are deprecated aliases of the two-arm and N-arm shapes. They forward here unchanged.

Examples

Example

import { when, vec2 } from 'typeshade'

const dir = when(
  segLen.lt(1e-6),
  () => vec2(1, 0),
  () => segVec.div(segLen),
)

const band = when(
  [
    [level.lt(4), () => coarse],
    [level.lt(9), () => medium],
  ],
  () => fine,
)

In the guide

Source

src/core/ir/builder.ts, line 2012, at commit 26de7be8

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when()