# std/columns

A record type laid out as one array per field.

```vyrn
import { columns } from "std/columns"
import { BodyColumns, bodyColumns, pushBody, bodyAt } from columns("./bodies")

```

`columns` is a `gen fn` that reflects a module and writes, for each record type `T` it exports:

- `type TColumns`: one `Array<F>` per field `f: F` of `T`, under the same name, with a `where` rule that every column has the first one's length. `c.f[i]` reads a field and `c.f[i] = v` writes one in place.
- `tColumns(capacity)`: empty columns with room for `capacity` rows.
- `pushT(c, v)`: appends the row `v` to `c` in place. The pushes are one group of stores, so the rule is checked once, after the last column grows, and the check is proved.
- `tAt(c, i)`: row `i` read back as a `T`.

`c.f.length` counts the rows. Because of the rule, a loop bounded by one column's length indexes every column with no bounds check. A walk over one field of many rows reads only that field's bytes, and a layout of one array of records reads the whole row.

## columns

`fn`, at line 28.

```vyrn
gen fn columns(module: String) -> String
```

Emits a module exporting the column layout of every record type `module` exports.

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From https://vyrn-lang.github.io/docs/std/columns.html — generated by `site/export.vyrn` from the Vyrn repository.