From 5d7697a8438e2ec6ce7f2e9a231cc127e0751ef0 Mon Sep 17 00:00:00 2001
From: Nic Crane <thisisnic@gmail.com>
Date: Thu, 28 Sep 2023 21:40:45 +0100
Subject: [PATCH] GH-37842: [R] Implement infer_schema.data.frame() (#37843)

### Rationale for this change

Users will be able to easily see the schema which their `data.frame` object will have when it's converted into an Arrwo table.

### What changes are included in this PR?

Implements `infer_schema()` method for `data.frame` objects.

Before:

``` r
library(arrow)
schema(mtcars)
#> Error in UseMethod("infer_schema"): no applicable method for 'infer_schema' applied to an object of class "data.frame"
```
After:

``` r
library(arrow)
schema(mtcars)
#> Schema
#> mpg: double
#> cyl: double
#> disp: double
#> hp: double
#> drat: double
#> wt: double
#> qsec: double
#> vs: double
#> am: double
#> gear: double
#> carb: double
#>
#> See $metadata for additional Schema metadata
```

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes
* Closes: #37842

Authored-by: Nic Crane <thisisnic@gmail.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>
---
 r/NAMESPACE                    | 1 +
 r/R/schema.R                   | 3 +++
 r/tests/testthat/test-schema.R | 5 +++++
 3 files changed, 9 insertions(+)

diff --git a/r/NAMESPACE b/r/NAMESPACE
index 21f88b4180d24..d49255f781f94 100644
--- a/r/NAMESPACE
+++ b/r/NAMESPACE
@@ -112,6 +112,7 @@ S3method(infer_schema,ArrowTabular)
 S3method(infer_schema,Dataset)
 S3method(infer_schema,RecordBatchReader)
 S3method(infer_schema,arrow_dplyr_query)
+S3method(infer_schema,data.frame)
 S3method(infer_type,ArrowDatum)
 S3method(infer_type,Expression)
 S3method(infer_type,blob)
diff --git a/r/R/schema.R b/r/R/schema.R
index 1ad18e314191e..ac0604b2b345c 100644
--- a/r/R/schema.R
+++ b/r/R/schema.R
@@ -285,6 +285,9 @@ infer_schema.Dataset <- function(x) x$schema
 #' @export
 infer_schema.arrow_dplyr_query <- function(x) implicit_schema(x)
 
+#' @export
+infer_schema.data.frame <- function(x) schema(!!!lapply(x, infer_type))
+
 #' @export
 names.Schema <- function(x) x$names
 
diff --git a/r/tests/testthat/test-schema.R b/r/tests/testthat/test-schema.R
index db91cee330960..b1dc06592955e 100644
--- a/r/tests/testthat/test-schema.R
+++ b/r/tests/testthat/test-schema.R
@@ -295,9 +295,14 @@ test_that("schema name assignment", {
 
 test_that("schema extraction", {
   skip_if_not_available("dataset")
+
   tbl <- arrow_table(example_data)
+  expect_equal(schema(example_data), tbl$schema)
   expect_equal(schema(tbl), tbl$schema)
 
+  expect_equal(schema(data.frame(a = 1, a = "x", check.names = FALSE)), schema(a = double(), a = string()))
+  expect_equal(schema(data.frame()), schema())
+
   ds <- InMemoryDataset$create(example_data)
   expect_equal(schema(ds), ds$schema)