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echo" IDENTICAL OUTPUT: $display_name produces a byte-identical executable on both binaries; treating the timing difference as a false positive (measurement or binary-layout noise), not a regression."
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echo""
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return 0
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fi
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echo" SLOWER EXECUTION CONFIRMED in $display_name (${pct_change}% / ${abs_delta_ms} ms then ${confirm_pct_change}% / ${confirm_abs_delta_ms} ms)"
Copy file name to clipboardExpand all lines: docs/langref/loops.md
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@@ -4,16 +4,37 @@ Loops let you run the same code multiple times, in...well, in a loop.
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## `for` Loops
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A `for` loop iterates over each item in a list:
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A `for` loop lets you run code on each item in an [iterator](iterators). For example:
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```roc
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var $sum = 0
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for n in 1.to(5) {
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$sum = $sum + n
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}
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```
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> The `to` method returns a [range](numbers#ranges) which is an `Iter` of the number in question. For example, [`I64.to`](builtins/I64#to) returns `Iter(I64)`, and so `n` in this example would be an `I64`.
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A loop body only includes statements; it does not have a final expression. The loop itself evaluates to `{}`.
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### Iterating over types that have `iter`
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`for` can also be used on types that have an `iter` method, as long as that method returns an [`Iter`](builtins/Iter). For example, [`List`](builtins/list) has (`List.iter`)[builtins/List#iter], so you can do a `for` loop over a list:
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```roc
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var $sum = 0
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for n in [1, 2, 3, 4] {
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$sum = $sum + n
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}
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```
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The item can be destructured inline:
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At runtime, this `[1, 2, 3, 4]` code snippet is exactly as efficient as the earlier `1.to(5)` one. In one case, `1.to(5)` will be evaluated to an `Iter` at compile time, and in the other, `[1, 2, 3, 4].iter()` will be evaluated at compile time to an identical `Iter`. By the time either program actually runs, they will have the same memory contents and will be executing the same instructions.
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### Pattern matching in `for`
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Whatever you put between `for` and `in` is treated as a [pattern](pattern-matching), meaning (for example) that the item can be destructured inline:
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```roc
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var $total = 0
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}
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```
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Use `_` if you don't want to name the item:
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As usual, you can nest patterns as much as you like, and can use `_` if you don't want to name a pattern:
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```roc
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var $count = 0
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}
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```
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A loop body only includes statements; it does not have a final expression. The loop itself evaluates to `{}`.
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Just like with [assignments](statements#assignments), the pattern you use here must be [exhaustive](pattern-matching#exhaustiveness). For example, the following would give an exhaustiveness error because the loop body couldn't know what value to use for `amount_to_add` if the item was ever `Err` at runtime:
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```roc
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var $count = 0
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for Ok(amount_to_add) in items {
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$count = $count + amount_to_add
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}
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```
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If you can't write an exhaustive pattern-match, you can name the entire iterator item and then use [`match`](pattern-matching#match) on it inside the loop body.
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