Question

好的，这里有个奇怪的问题。我正在使用FSharp.Data.SqlClient从我们的数据库中获取记录。它推断的记录有几个字段，它们是选项类型。我需要过滤掉任何选项类型为None的记录，并创建已知字段的新记录。以下是我所谈论的一个例子。为了解决这个问题，我创建了一个过滤函数recordFilter，它在所有类型的Option<'T>包含值时返回我想要的类型，而在None不包含Option<'T>时返回Option<'T>。

我的问题是，是否可以创建一个仅自动检查记录中的所有match...with字段以获取值的函数。我猜这需要某种反射来迭代记录的字段。我猜这是不可能的，但我想把它扔出去，万一我错了。

如果这种方法是惯用的方式，那么我很乐意听到这一点。我只是想确保我没有错过一些更优雅的解决方案。 F＃的可能性让我感到惊讶。

我的动机是我正在处理包含type OptionRecord = { Id: int Attr1: int option Attr2: int option Attr3: int option Attr4: int option Attr5: int option Attr6: int option } type FilteredRecord = { Id: int Attr1: int Attr2: int Attr3: int Attr4: int Attr5: int Attr6: int } let optionRecords = [for i in 1..5 -> { OptionRecord.Id = i Attr1 = Some i Attr2 = match i % 2 = 0 with | true -> Some i | false -> None Attr3 = Some i Attr4 = Some i Attr5 = Some i Attr6 = Some i }] let recordFilter (x:OptionRecord) = match x.Attr1, x.Attr2, x.Attr3, x.Attr4, x.Attr5, x.Attr6 with | Some attr1, Some attr2, Some attr3, Some attr4, Some attr5, Some attr6 -> Some { FilteredRecord.Id = x.Id Attr1 = attr1 Attr2 = attr2 Attr3 = attr3 Attr4 = attr4 Attr5 = attr5 Attr6 = attr6 } | _, _, _, _, _, _ -> None let filteredRecords = optionRecords |> List.choose recordFilter类型的数十个字段的记录。像我在这个例子中那样写出一个巨大的//Convert bitmap to byte array Bitmap bitmap = mResultsBitmap; ByteArrayOutputStream bos = new ByteArrayOutputStream(); bitmap.compress(Bitmap.CompressFormat.PNG, 0 /*ignored for PNG*/, bos); byte[] bitmapdata = bos.toByteArray(); //write the bytes in file FileOutputStream fos = null; try { fos = new FileOutputStream(f); } catch (FileNotFoundException e) { e.printStackTrace(); } try { fos.write(bitmapdata); } catch (IOException e) { e.printStackTrace(); } try { fos.flush(); } catch (IOException e) { e.printStackTrace(); } try { fos.close(); } catch (IOException e) { e.printStackTrace(); } final File pFIle2 = f; //TODO: clarifai stuff //TODO: clarifai stuff Log.e("this:"," this is running 0"); Thread thread = new Thread(new Runnable() { @Override public void run() { Log.e("this:", " this is running 1"); client = new ClarifaiBuilder("mykeyhere1234}").buildSync(); Log.e("this:", " this is running 2"); Thread th = new Thread(new Runnable() { @Override public void run() { Log.e("this:", " this is running 3"); Log.e("this", client.getDefaultModels().generalModel().predict() .withInputs( ClarifaiInput.forImage(ClarifaiImage.of(pFIle2)) ) .executeSync().rawBody()); Log.e("this:", " this is running 4"); } }); } });语句是很烦人的。当它只有几个字段是好的，当它是30多个字段时，它很烦人。

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Answer 1

这确实可以用反射来完成。命名空间FSharp.Reflection包含一些方便的帮助程序，专门用于F＃类型，而不是一般的.NET。需要考虑的关键点是：

FSharpType.GetRecordFields返回每个记录字段的PropertyInfo个对象列表。
您可以通过将其类型与option进行比较来判断该属性是否为typedefof<option>。
None在运行时表示为null。
FSharpValue.GetUnionFields和FSharpValue.GetRecordFields分别返回联合或记录字段值的列表。
FSharpValue.MakeRecord根据字段值列表创建新记录。

以下是代码：

open FSharp.Reflection

/// Record with Option-typed fields
type RM = { a: int option; b: string option; c: bool option }

/// Record with same fields, but non-optional
type R = { a: int; b: string; c: bool }

/// Determines if the given property is of type option<_>
let isOption (f: System.Reflection.PropertyInfo) = 
    f.PropertyType.IsGenericType && f.PropertyType.GetGenericTypeDefinition() = typedefof<option<_>>

/// Returns an array of pairs (propertyInfo, value) for every field of the given record.
let fieldsWithValues (r: 'a) =
    Array.zip (FSharpType.GetRecordFields typeof<'a>) (FSharpValue.GetRecordFields r)

/// Determines if the given record has any option-type fields whose value is None.
let anyNones (r: 'a) = 
    fieldsWithValues r |> Seq.exists (fun (f, value) -> isOption f && isNull value)

/// Given two records, 'a and 'b, where 'a is expected to contain some option-typed
/// fields, and 'b is expected to contain their non-option namesakes, creates a new
/// record 'b with all non-None option values copied from 'a.
let copyOptionFields (from: 'a) (to': 'b) : 'b =
    let bFields = FSharpValue.GetRecordFields to'
    let aFields = Array.zip (FSharpType.GetRecordFields typeof<'a>) (FSharpValue.GetRecordFields from)
    for idx, (f, value) in aFields |> Array.indexed do
        if isOption f && not (isNull value) then
            let _, values = FSharpValue.GetUnionFields( value, f.PropertyType )
            bFields.[idx] <- values.[0] // We know that this is a `Some` case, and it has only one value

    FSharpValue.MakeRecord( typeof<'b>, bFields ) :?> 'b

用法：

> anyNones {RM.a = Some 42; b = Some "abc"; c = Some true} 
val it : bool = false

> anyNones {RM.a = Some 42; b = Some "abc"; c = None}
val it : bool = true

> let emptyR = {R.a = 0; b = ""; c = false}

> copyOptionFields {RM.a = Some 42; b = Some "abc"; c = Some true} emptyR
val it : R = {a = 42; b = "abc"; c = true;}

> copyOptionFields {RM.a = None; b = Some "abc"; c = None} emptyR
val it : R = {a = 0; b = "abc"; c = false;}

注意：上面的代码不执行任何健全性检查（例如，'a和'b确实是记录，或者他们的字段确实是同名的并且在同一个订单等）。我将此作为练习留给读者： - ）

注意2 ：小心表现。由于这是反射，因此速度较慢，无法在编译时进行优化。

F＃过滤包含多种选项类型的记录

1 个答案: