Fix Float mapping with downcasting (#142)

The behavior of `NSNumber as? Float` in Swift is such that some values that cannot "safely express the value stored" in NSNumber. This means only using `as?` to cast to `Float` can fail, where doing `.floatValue` or `Float.init` would succeed. To work around this oddity, this new `Convertible` conformance calls `.floatValue` on `NSNumber`, leading to a float, that is possibly less precise than you'd like.
lyft · Jul 31, 2018 · 4adc773 · 4adc773
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diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -6,7 +6,9 @@
 
 ## Enhancements
 
-- None
+- Fix `Float` behavior for some values
+  [Keith Smiley](https://github.com/keith)
+  [#142](https://github.com/lyft/mapper/pull/142)
 
 # 8.0.0
 

diff --git a/Mapper.xcodeproj/project.pbxproj b/Mapper.xcodeproj/project.pbxproj
@@ -18,6 +18,7 @@
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 		C207F6EC1D7F286700EBCC74 /* TransformTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = C207F6E31D7F286700EBCC74 /* TransformTests.swift */; };
+		C21F9CCE210FCF5F00761DDC /* Float+Convertible.swift in Sources */ = {isa = PBXBuildFile; fileRef = C21F9CCD210FCF5F00761DDC /* Float+Convertible.swift */; };
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 		C2C036FA1C2B1A0B003FB853 /* Convertible.swift in Sources */ = {isa = PBXBuildFile; fileRef = C2C036F31C2B1A0B003FB853 /* Convertible.swift */; };
@@ -52,6 +53,7 @@
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 		C207F6E31D7F286700EBCC74 /* TransformTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = TransformTests.swift; sourceTree = "<group>"; };
+		C21F9CCD210FCF5F00761DDC /* Float+Convertible.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "Float+Convertible.swift"; sourceTree = "<group>"; };
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@@ -139,6 +141,7 @@
 			children = (
 				C2C036F31C2B1A0B003FB853 /* Convertible.swift */,
 				C20586EB1CDEAD9900658A67 /* DefaultConvertible.swift */,
+				C21F9CCD210FCF5F00761DDC /* Float+Convertible.swift */,
 				C2C036F51C2B1A0B003FB853 /* Mappable.swift */,
 				C2C036F61C2B1A0B003FB853 /* Mapper.swift */,
 				C2C036F41C2B1A0B003FB853 /* MapperError.swift */,
@@ -236,6 +239,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				C21F9CCE210FCF5F00761DDC /* Float+Convertible.swift in Sources */,
 				C2C036FF1C2B1A0B003FB853 /* Transform+Dictionary.swift in Sources */,
 				C2C036FE1C2B1A0B003FB853 /* URL+Convertible.swift in Sources */,
 				C2B2A5761D3DE82700F7E7DE /* NSDictionary+Safety.swift in Sources */,

diff --git a/Sources/DefaultConvertible.swift b/Sources/DefaultConvertible.swift
@@ -35,6 +35,5 @@ extension Int64: DefaultConvertible {}
 extension UInt: DefaultConvertible {}
 extension UInt32: DefaultConvertible {}
 extension UInt64: DefaultConvertible {}
-extension Float: DefaultConvertible {}
 extension Double: DefaultConvertible {}
 extension Bool: DefaultConvertible {}
diff --git a/Sources/Float+Convertible.swift b/Sources/Float+Convertible.swift
@@ -0,0 +1,16 @@
+import Foundation
+
+// Float convertible implementation
+extension Float: Convertible {
+    public static func fromMap(_ value: Any) throws -> Float {
+        if let value = value as? Float {
+            return value
+        }
+
+        if let object = value as? NSNumber {
+            return object.floatValue
+        }
+
+        throw MapperError.convertibleError(value: value, type: Float.self)
+    }
+}
diff --git a/Tests/MapperTests/ConvertibleValueTests.swift b/Tests/MapperTests/ConvertibleValueTests.swift
@@ -237,4 +237,22 @@ final class ConvertibleValueTests: XCTestCase {
         let test = Test.from(["foo": "not a dictionary"])
         XCTAssertNil(test)
     }
+
+    func testFloatDirectly() throws {
+        let float: Float = 1.1
+        let value = try Float.fromMap(float)
+        XCTAssertEqual(value, 1.1)
+    }
+
+    func testConvertingFloat() throws {
+        struct Test: Mappable {
+            let float: Float
+            init(map: Mapper) throws {
+                try self.float = map.from("float")
+            }
+        }
+
+        let test = try Test(map: Mapper(JSON: ["float": 1.1]))
+        XCTAssertEqual(test.float, 1.1)
+    }
 }