How to embed for loops in Kotlin string templates

Question

We can easily nest expression operators like if and when in Kotlin string templates:

"List ${if (list.isEmpty()) "is empty" else "has ${list.size} items"}."

But for or while are not expressions and can't be nested in template like this:

"<ol>${for (item in list) "<li>$item"}</ol>"

So I was looking for convinient ways to use loops inside large templates.

Answer 1

The easiest out-of-the-box way I found so far is to replace loops with equivalent joinToString calls:

"<ol>${list.joinToString("") { "<li>$it" }}</ol>"

or

"""
<ol>${list.indices.joinToString("") {
    """
    <li id="item${it + 1}">${list[it]}"""
}}
</ol>""".trimIndent()

In the matter of preference, it's also possible to simulate loops with helper functions:

inline fun <T> forEach(iterable: Iterable<T>, crossinline out: (v: T) -> String) 
    = iterable.joinToString("") { out(it) }

fun <T> forEachIndexed1(iterable: Iterable<T>, out: (i: Int, v: T) -> String): String {
    val sb = StringBuilder()
    iterable.forEachIndexed { i, it ->
        sb.append(out(i + 1, it))
    }
    return sb.toString()
}

and use them like this:

"<ol>${forEach(list) { "<li>$it" }}</ol>"

or

"""
<ol>${forEachIndexed1(list) { i, item ->
"""
    <li id="item$i">$item"""
}}
</ol>""".trimIndent()