UseSingleAssertion

Issue ID UseSingleAssertion · Severity Warning · Category Productivity · Default on

Flags the assertk.all { hasSize(1); first()... } (and the index(0) equivalent) pattern where assertk's single() should be used.

Why

Assert<List<T>>.single() asserts that the collection has exactly one element and transforms the subject to that element. Doing it by hand — hasSize(1) plus first()/index(0) inside an all { } block — is two assertions where one will do, and the chained form reads more naturally.

The detector fires only on the simple two-statement shape inside an all { } block from assertk:

• hasSize(1) paired with first().<something>, or
• hasSize(1) paired with index(0).<something>.

The order doesn't matter. Anything else — hasSize(2), index(1), extra statements in the block — is left alone.

Example

FlaggedPreferred

import assertk.all
import assertk.assertThat
import assertk.assertions.hasSize
import assertk.assertions.index
import assertk.assertions.isEqualTo

fun onlyElement() {
    val list: List<String> = listOf("a")
    assertThat(list).all {
        hasSize(1)
        index(0).isEqualTo("a")
    }
}

import assertk.assertThat
import assertk.assertions.isEqualTo
import assertk.assertions.single

fun onlyElement() {
    val list: List<String> = listOf("a")
    assertThat(list).single().isEqualTo("a")
}

The first() form is recognised the same way:

assertThat(list).all {
    hasSize(1)
    first().isEqualTo("a") // also flagged
}

Quick fix

Yes — the IDE collapses the entire all { } block to single().<chained assertion>, preserving whatever assertion was applied to the element, and adds the assertk.assertions.single import.

Source

SingleAssertionDetector.kt