Add expect/to_loosely_equal

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Added `expect/to_loosely_equal` for asserting on `Float` values.
+
 ## [0.2.2] - 2024-05-06
 
 ### Changed

src/startest/expect.gleam

@@ -1,6 +1,7 @@
 //// Assertions to be used in tests.
 
 import exception
+import gleam/float
 import gleam/option.{type Option, None, Some}
 import gleam/string
 import startest/assertion_error.{AssertionError}
@@ -138,3 +139,29 @@ pub fn to_not_throw(f: fn() -> a) -> a {
       |> assertion_error.raise
   }
 }
+
+/// Asserts that the given `Float` is equal to the expected `Float` within the
+/// provided tolerance.
+pub fn to_loosely_equal(
+  actual: Float,
+  expected expected: Float,
+  tolerating tolerance: Float,
+) -> Nil {
+  case float.loosely_equals(actual, expected, tolerance) {
+    True -> Nil
+    False ->
+      AssertionError(
+        string.concat([
+          "Expected ",
+          string.inspect(actual),
+          " to loosely equal ",
+          string.inspect(expected),
+          " with a tolerance of ",
+          string.inspect(tolerance),
+        ]),
+        string.inspect(actual),
+        string.inspect(expected) <> " ± " <> string.inspect(tolerance),
+      )
+      |> assertion_error.raise
+  }
+}

test/startest/expect_test.gleam

@@ -192,3 +192,22 @@ pub fn to_not_throw_tests() {
     ]),
   ])
 }
+
+pub fn to_loosely_equal_tests() {
+  describe("startest/expect", [
+    describe("to_loosely_equal", [
+      describe("given two loosely equal floats", [
+        it_passes(fn() {
+          2.0
+          |> expect.to_loosely_equal(2.4, tolerating: 0.5)
+        }),
+      ]),
+      describe("given two loosely inequal floats", [
+        it_fails(fn() {
+          2.0
+          |> expect.to_loosely_equal(2.4, tolerating: 0.1)
+        }),
+      ]),
+    ]),
+  ])
+}