然而，我在这里寻找的是有用的片段，可重用的小“帮助”功能。或者你永远记不住的模糊但漂亮的模式。
就像是：
open System.IO
let rec visitor dir filter=
seq { yield! Directory.GetFiles(dir, filter)
for subdir in Directory.GetDirectories(dir) do
yield! visitor subdir filter}
我想让它成为一种方便的参考页面。因此，不会有正确的答案，但希望有很多好的答案。
编辑Tomas Petricek 专门为 F# 片段http://fssnip.net/创建了一个站点。
Perl 风格的正则表达式匹配
let (=~) input pattern =
System.Text.RegularExpressions.Regex.IsMatch(input, pattern)
let test = "monkey" =~ "monk.+"它允许您使用符号匹配文本。
中缀运算符
我从http://sandersn.com/blog//index.php/2009/10/22/infix-function-trick-for-f得到这个，去那个页面了解更多细节。
如果您了解 Haskell，您可能会发现自己在 F# 中缺少中缀糖：
// standard Haskell call has function first, then args just like F#. So obviously
// here there is a function that takes two strings: string -> string -> string
startsWith "kevin" "k"
//Haskell infix operator via backQuotes. Sometimes makes a function read better.
"kevin" `startsWith` "K"
虽然 F# 没有真正的“中缀”运算符，但同样的事情可以通过管道和“后管道”几乎同样优雅地完成（谁知道这样的事情？？）
// F# 'infix' trick via pipelines
"kevin" |> startsWith <| "K"
多行字符串
这是非常微不足道的，但它似乎是 F# 字符串的一个不广为人知的特性。
let sql = "select a,b,c \
from table \
where a = 1"
这会产生：
val sql : string = "select a,b,c from table where a = 1"
当 F# 编译器在字符串文字中看到反斜杠后跟回车时，它将删除从反斜杠到下一行第一个非空格字符的所有内容。这允许您拥有排列的多行字符串文字，而无需使用一堆字符串连接。
通用记忆，由该人本人提供
let memoize f =
let cache = System.Collections.Generic.Dictionary<_,_>(HashIdentity.Structural)
fun x ->
let ok, res = cache.TryGetValue(x)
if ok then res
else let res = f x
cache.[x] <- res
res
使用它，您可以像这样执行缓存读取器：
let cachedReader = memoize reader
对文本文件的简单读写
这些是微不足道的，但使文件访问可管道化：
open System.IO
let fileread f = File.ReadAllText(f)
let filewrite f s = File.WriteAllText(f, s)
let filereadlines f = File.ReadAllLines(f)
let filewritelines f ar = File.WriteAllLines(f, ar)
所以
let replace f (r:string) (s:string) = s.Replace(f, r)
"C:\\Test.txt" |>
fileread |>
replace "teh" "the" |>
filewrite "C:\\Test.txt"
并将其与问题中引用的访问者相结合：
let filereplace find repl path =
path |> fileread |> replace find repl |> filewrite path
let recurseReplace root filter find repl =
visitor root filter |> Seq.iter (filereplace find repl)
如果您希望能够读取“锁定”文件（例如，已经在 Excel 中打开的 csv 文件......），请稍作改进：
let safereadall f =
use fs = new FileStream(f, FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
use sr = new StreamReader(fs, System.Text.Encoding.Default)
sr.ReadToEnd()
let split sep (s:string) = System.Text.RegularExpressions.Regex.Split(s, sep)
let fileread f = safereadall f
let filereadlines f = f |> safereadall |> split System.Environment.NewLine
对于需要检查 null 的性能密集型内容
let inline isNull o = System.Object.ReferenceEquals(o, null)
if isNull o then ... else ...
大约快 20 倍
if o = null then ... else ...
Active Patterns，又名“Banana Splits”，是一种非常方便的构造，可以让一个匹配多个正则表达式模式。这很像AWK ，但没有DFA的高性能，因为模式是按顺序匹配的，直到成功。
#light
open System
open System.Text.RegularExpressions
let (|Test|_|) pat s =
if (new Regex(pat)).IsMatch(s)
then Some()
else None
let (|Match|_|) pat s =
let opt = RegexOptions.None
let re = new Regex(pat,opt)
let m = re.Match(s)
if m.Success
then Some(m.Groups)
else None
一些使用示例：
let HasIndefiniteArticle = function
| Test "(?: |^)(a|an)(?: |$)" _ -> true
| _ -> false
type Ast =
| IntVal of string * int
| StringVal of string * string
| LineNo of int
| Goto of int
let Parse = function
| Match "^LET\s+([A-Z])\s*=\s*(\d+)$" g ->
IntVal( g.[1].Value, Int32.Parse(g.[2].Value) )
| Match "^LET\s+([A-Z]\$)\s*=\s*(.*)$" g ->
StringVal( g.[1].Value, g.[2].Value )
| Match "^(\d+)\s*:$" g ->
LineNo( Int32.Parse(g.[1].Value) )
| Match "^GOTO \s*(\d+)$" g ->
Goto( Int32.Parse(g.[1].Value) )
| s -> failwithf "Unexpected statement: %s" s
'Unitize' 一个不处理单元
的函数使用FloatWithMeasure函数http://msdn.microsoft.com/en-us/library/ee806527(VS.100).aspx。
let unitize (f:float -> float) (v:float<'u>) =
LanguagePrimitives.FloatWithMeasure<'u> (f (float v))
例子：
[<Measure>] type m
[<Measure>] type kg
let unitize (f:float -> float) (v:float<'u>) =
LanguagePrimitives.FloatWithMeasure<'u> (f (float v))
//this function doesn't take units
let badinc a = a + 1.
//this one does!
let goodinc v = unitize badinc v
goodinc 3.<m>
goodinc 3.<kg>
旧版本：
let unitize (f:float -> float) (v:float<'u>) =
let unit = box 1. :?> float<'u>
unit * (f (v/unit))
也许单子
type maybeBuilder() =
member this.Bind(v, f) =
match v with
| None -> None
| Some(x) -> f x
member this.Delay(f) = f()
member this.Return(v) = Some v
let maybe = maybeBuilder()
这是一个简单的单子介绍。
选项合并运算符
我想要一个defaultArg语法更接近 C# null-coalescing 运算符的函数版本，??. 这让我可以使用非常简洁的语法从选项中获取值，同时提供默认值。
/// Option-coalescing operator - this is like the C# ?? operator, but works with
/// the Option type.
/// Warning: Unlike the C# ?? operator, the second parameter will always be
/// evaluated.
/// Example: let foo = someOption |? default
let inline (|?) value defaultValue =
defaultArg value defaultValue
/// Option-coalescing operator with delayed evaluation. The other version of
/// this operator always evaluates the default value expression. If you only
/// want to create the default value when needed, use this operator and pass
/// in a function that creates the default.
/// Example: let foo = someOption |?! (fun () -> new Default())
let inline (|?!) value f =
match value with Some x -> x | None -> f()
比例/比率函数生成器
同样，微不足道，但很方便。
//returns a function which will convert from a1-a2 range to b1-b2 range
let scale (a1:float<'u>, a2:float<'u>) (b1:float<'v>,b2:float<'v>) =
let m = (b2 - b1)/(a2 - a1) //gradient of line (evaluated once only..)
(fun a -> b1 + m * (a - a1))
例子：
[<Measure>] type m
[<Measure>] type px
let screenSize = (0.<px>, 300.<px>)
let displayRange = (100.<m>, 200.<m>)
let scaleToScreen = scale displayRange screenSize
scaleToScreen 120.<m> //-> 60.<px>
转置列表（参见Jomo Fisher 的博客）
///Given list of 'rows', returns list of 'columns'
let rec transpose lst =
match lst with
| (_::_)::_ -> List.map List.head lst :: transpose (List.map List.tail lst)
| _ -> []
transpose [[1;2;3];[4;5;6];[7;8;9]] // returns [[1;4;7];[2;5;8];[3;6;9]]
这是一个尾递归版本，它（来自我的粗略分析）稍微慢一些，但是当内部列表长于 10000 个元素（在我的机器上）时，它的优点是不会引发堆栈溢出：
let transposeTR lst =
let rec inner acc lst =
match lst with
| (_::_)::_ -> inner (List.map List.head lst :: acc) (List.map List.tail lst)
| _ -> List.rev acc
inner [] lst
如果我很聪明，我会尝试将它与异步并行化......
（我知道，我知道，System.Collections.Generic.Dictionary 并不是真正的“C#”字典）
C# 到 F#
(dic :> seq<_>) //cast to seq of KeyValuePair
|> Seq.map (|KeyValue|) //convert KeyValuePairs to tuples
|> Map.ofSeq //convert to Map
（来自 Brian，here，Mauricio 在下面的评论中提出了改进。(|KeyValue|)是匹配 KeyValuePair 的活动模式 - 来自 FSharp.Core - 相当于(fun kvp -> kvp.Key, kvp.Value)）
有趣的选择
要获得所有不可变的优点，但使用 Dictionary 的 O(1) 查找速度，您可以使用dict返回不可变 IDictionary 的运算符（请参阅此问题）。
我目前看不到使用此方法直接转换字典的方法，除了
(dic :> seq<_>) //cast to seq of KeyValuePair
|> (fun kvp -> kvp.Key, kvp.Value) //convert KeyValuePairs to tuples
|> dict //convert to immutable IDictionary
F# 到 C#
let dic = Dictionary()
map |> Map.iter (fun k t -> dic.Add(k, t))
dic
这里奇怪的是 FSI 会将类型报告为（例如）：
val it : Dictionary<string,int> = dict [("a",1);("b",2)]
但如果您反馈dict [("a",1);("b",2)]，FSI 会报告
IDictionary<string,int> = seq[[a,1] {Key = "a"; Value = 1; } ...
LINQ-to-XML 帮助程序
namespace System.Xml.Linq
// hide warning about op_Explicit
#nowarn "77"
[<AutoOpen>]
module XmlUtils =
/// Converts a string to an XName.
let xn = XName.op_Implicit
/// Converts a string to an XNamespace.
let xmlns = XNamespace.op_Implicit
/// Gets the string value of any XObject subclass that has a Value property.
let inline xstr (x : ^a when ^a :> XObject) =
(^a : (member get_Value : unit -> string) x)
/// Gets a strongly-typed value from any XObject subclass, provided that
/// an explicit conversion to the output type has been defined.
/// (Many explicit conversions are defined on XElement and XAttribute)
/// Example: let value:int = xval foo
let inline xval (x : ^a when ^a :> XObject) : ^b =
((^a or ^b) : (static member op_Explicit : ^a -> ^b) x)
/// Dynamic lookup operator for getting an attribute value from an XElement.
/// Returns a string option, set to None if the attribute was not present.
/// Example: let value = foo?href
/// Example with default: let value = defaultArg foo?Name "<Unknown>"
let (?) (el:XElement) (name:string) =
match el.Attribute(xn name) with
| null -> None
| att -> Some(att.Value)
/// Dynamic operator for setting an attribute on an XElement.
/// Example: foo?href <- "http://www.foo.com/"
let (?<-) (el:XElement) (name:string) (value:obj) =
el.SetAttributeValue(xn name, value)
树排序/将树展平为列表
我有以下二叉树：
___ 77 _
/ \
______ 47 __ 99
/ \
21 _ 54
\ / \
43 53 74
/
39
/
32
表示如下：
type 'a tree =
| Node of 'a tree * 'a * 'a tree
| Nil
let myTree =
Node
(Node
(Node (Nil,21,Node (Node (Node (Nil,32,Nil),39,Nil),43,Nil)),47,
Node (Node (Nil,53,Nil),54,Node (Nil,74,Nil))),77,Node (Nil,99,Nil))
扁平化树的一种简单方法是：
let rec flatten = function
| Nil -> []
| Node(l, a, r) -> flatten l @ a::flatten r
这不是尾递归，我相信@操作员会导致它成为 O(n log n) 或 O(n^2) 与不平衡的二叉树。稍作调整，我想出了这个尾递归 O(n) 版本：
let flatten2 t =
let rec loop acc c = function
| Nil -> c acc
| Node(l, a, r) ->
loop acc (fun acc' -> loop (a::acc') c l) r
loop [] (fun x -> x) t
这是 fsi 中的输出：
> flatten2 myTree;;
val it : int list = [21; 32; 39; 43; 47; 53; 54; 74; 77; 99]
Performance testing
(Found here and updated for latest release of F#)
open System
open System.Diagnostics
module PerformanceTesting =
let Time func =
let stopwatch = new Stopwatch()
stopwatch.Start()
func()
stopwatch.Stop()
stopwatch.Elapsed.TotalMilliseconds
let GetAverageTime timesToRun func =
Seq.initInfinite (fun _ -> (Time func))
|> Seq.take timesToRun
|> Seq.average
let TimeOperation timesToRun =
GC.Collect()
GetAverageTime timesToRun
let TimeOperations funcsWithName =
let randomizer = new Random(int DateTime.Now.Ticks)
funcsWithName
|> Seq.sortBy (fun _ -> randomizer.Next())
|> Seq.map (fun (name, func) -> name, (TimeOperation 100000 func))
let TimeOperationsAFewTimes funcsWithName =
Seq.initInfinite (fun _ -> (TimeOperations funcsWithName))
|> Seq.take 50
|> Seq.concat
|> Seq.groupBy fst
|> Seq.map (fun (name, individualResults) -> name, (individualResults |> Seq.map snd |> Seq.average))
数组的加权和
基于 [k-array] 权重计算 [k-array of n-arrays] 数字的加权 [n-array] 总和
（从this question和kvb的答案复制）
鉴于这些数组
let weights = [|0.6;0.3;0.1|]
let arrs = [| [|0.0453;0.065345;0.07566;1.562;356.6|] ;
[|0.0873;0.075565;0.07666;1.562222;3.66|] ;
[|0.06753;0.075675;0.04566;1.452;3.4556|] |]
我们想要一个加权和（按列），因为数组的两个维度都可以是可变的。
Array.map2 (fun w -> Array.map ((*) w)) weights arrs
|> Array.reduce (Array.map2 (+))
第一行：第一个 Array.map2 函数对权重的部分应用产生了一个新函数 (Array.map ((*) weight)，该函数（对于每个权重）应用于 arr 中的每个数组。
第二行：Array.reduce 类似于 fold，除了它从第二个值开始并使用第一个作为初始“状态”。在这种情况下，每个值都是我们数组的“行”。因此，在前两行应用 Array.map2 (+) 意味着我们将前两个数组相加，这给我们留下了一个新数组，然后我们 (Array.reduce) 再次将其相加到下一个（在本例中为最后一个）大批。
结果：
[|0.060123; 0.069444; 0.07296; 1.5510666; 215.40356|]
好的，这与片段无关，但我一直忘记这一点：
如果您在交互式窗口中，您点击F7跳回代码窗口（不取消选择您刚刚运行的代码......）
从代码窗口转到 F# 窗口（以及打开 F# 窗口）是Ctrl Alt F
（除非 CodeRush 偷了你的绑定......）
F#、DataReaders 的 DataSetExtensions
System.Data.DataSetExtensions.dll添加了将 aDataTable视为 aIEnumerable<DataRow>以及以DBNull通过支持 System.Nullable 来优雅处理的方式对单个单元格的值进行拆箱的能力。例如，在 C# 中，我们可以获取包含空值的整数列的值，并使用DBNull非常简洁的语法指定默认为零：
var total = myDataTable.AsEnumerable()
.Select(row => row.Field<int?>("MyColumn") ?? 0)
.Sum();
但是，有两个领域缺少 DataSetExtensions。一是不支持IDataReader，二是不支持F#option类型。以下代码两者兼而有之 - 它允许将IDataReadera 视为 a seq<IDataRecord>，并且它可以从读取器或数据集中拆箱值，并支持 F# 选项或 System.Nullable。结合另一个 answer 中的选项合并运算符，这允许在使用 DataReader 时使用如下代码：
let total =
myReader.AsSeq
|> Seq.map (fun row -> row.Field<int option>("MyColumn") |? 0)
|> Seq.sum
也许忽略数据库空值的更惯用的 F# 方式是......
let total =
myReader.AsSeq
|> Seq.choose (fun row -> row.Field<int option>("MyColumn"))
|> Seq.sum
此外，下面定义的扩展方法可用于 F# 和 C#/VB。
open System
open System.Data
open System.Reflection
open System.Runtime.CompilerServices
open Microsoft.FSharp.Collections
/// Ported from System.Data.DatasetExtensions.dll to add support for the Option type.
[<AbstractClass; Sealed>]
type private UnboxT<'a> private () =
// This class generates a converter function based on the desired output type,
// and then re-uses the converter function forever. Because the class itself is generic,
// different output types get different cached converter functions.
static let referenceField (value:obj) =
if value = null || DBNull.Value.Equals(value) then
Unchecked.defaultof<'a>
else
unbox value
static let valueField (value:obj) =
if value = null || DBNull.Value.Equals(value) then
raise <| InvalidCastException("Null cannot be converted to " + typeof<'a>.Name)
else
unbox value
static let makeConverter (target:Type) methodName =
Delegate.CreateDelegate(typeof<Converter<obj,'a>>,
typeof<UnboxT<'a>>
.GetMethod(methodName, BindingFlags.NonPublic ||| BindingFlags.Static)
.MakeGenericMethod([| target.GetGenericArguments().[0] |]))
|> unbox<Converter<obj,'a>>
|> FSharpFunc.FromConverter
static let unboxFn =
let theType = typeof<'a>
if theType.IsGenericType && not theType.IsGenericTypeDefinition then
let genericType = theType.GetGenericTypeDefinition()
if typedefof<Nullable<_>> = genericType then
makeConverter theType "NullableField"
elif typedefof<option<_>> = genericType then
makeConverter theType "OptionField"
else
invalidOp "The only generic types supported are Option<T> and Nullable<T>."
elif theType.IsValueType then
valueField
else
referenceField
static member private NullableField<'b when 'b : struct and 'b :> ValueType and 'b:(new:unit -> 'b)> (value:obj) =
if value = null || DBNull.Value.Equals(value) then
Nullable<_>()
else
Nullable<_>(unbox<'b> value)
static member private OptionField<'b> (value:obj) =
if value = null || DBNull.Value.Equals(value) then
None
else
Some(unbox<'b> value)
static member inline Unbox =
unboxFn
/// F# data-related extension methods.
[<AutoOpen>]
module FsDataEx =
type System.Data.IDataReader with
/// Exposes a reader's current result set as seq<IDataRecord>.
/// Reader is closed when sequence is fully enumerated.
member this.AsSeq =
seq { use reader = this
while reader.Read() do yield reader :> IDataRecord }
/// Exposes all result sets in a reader as seq<seq<IDataRecord>>.
/// Reader is closed when sequence is fully enumerated.
member this.AsMultiSeq =
let rowSeq (reader:IDataReader) =
seq { while reader.Read() do yield reader :> IDataRecord }
seq {
use reader = this
yield rowSeq reader
while reader.NextResult() do
yield rowSeq reader
}
/// Populates a new DataSet with the contents of the reader. Closes the reader after completion.
member this.ToDataSet () =
use reader = this
let dataSet = new DataSet(RemotingFormat=SerializationFormat.Binary, EnforceConstraints=false)
dataSet.Load(reader, LoadOption.OverwriteChanges, [| "" |])
dataSet
type System.Data.IDataRecord with
/// Gets a value from the record by name.
/// DBNull and null are returned as the default value for the type.
/// Supports both nullable and option types.
member this.Field<'a> (fieldName:string) =
this.[fieldName] |> UnboxT<'a>.Unbox
/// Gets a value from the record by column index.
/// DBNull and null are returned as the default value for the type.
/// Supports both nullable and option types.
member this.Field<'a> (ordinal:int) =
this.GetValue(ordinal) |> UnboxT<'a>.Unbox
type System.Data.DataRow with
/// Identical to the Field method from DatasetExtensions, but supports the F# Option type.
member this.Field2<'a> (columnName:string) =
this.[columnName] |> UnboxT<'a>.Unbox
/// Identical to the Field method from DatasetExtensions, but supports the F# Option type.
member this.Field2<'a> (columnIndex:int) =
this.[columnIndex] |> UnboxT<'a>.Unbox
/// Identical to the Field method from DatasetExtensions, but supports the F# Option type.
member this.Field2<'a> (column:DataColumn) =
this.[column] |> UnboxT<'a>.Unbox
/// Identical to the Field method from DatasetExtensions, but supports the F# Option type.
member this.Field2<'a> (columnName:string, version:DataRowVersion) =
this.[columnName, version] |> UnboxT<'a>.Unbox
/// Identical to the Field method from DatasetExtensions, but supports the F# Option type.
member this.Field2<'a> (columnIndex:int, version:DataRowVersion) =
this.[columnIndex, version] |> UnboxT<'a>.Unbox
/// Identical to the Field method from DatasetExtensions, but supports the F# Option type.
member this.Field2<'a> (column:DataColumn, version:DataRowVersion) =
this.[column, version] |> UnboxT<'a>.Unbox
/// C# data-related extension methods.
[<Extension; AbstractClass; Sealed>]
type CsDataEx private () =
/// Populates a new DataSet with the contents of the reader. Closes the reader after completion.
[<Extension>]
static member ToDataSet(this:IDataReader) =
this.ToDataSet()
/// Exposes a reader's current result set as IEnumerable{IDataRecord}.
/// Reader is closed when sequence is fully enumerated.
[<Extension>]
static member AsEnumerable(this:IDataReader) =
this.AsSeq
/// Exposes all result sets in a reader as IEnumerable{IEnumerable{IDataRecord}}.
/// Reader is closed when sequence is fully enumerated.
[<Extension>]
static member AsMultipleEnumerable(this:IDataReader) =
this.AsMultiSeq
/// Gets a value from the record by name.
/// DBNull and null are returned as the default value for the type.
/// Supports both nullable and option types.
[<Extension>]
static member Field<'T> (this:IDataRecord, fieldName:string) =
this.Field<'T>(fieldName)
/// Gets a value from the record by column index.
/// DBNull and null are returned as the default value for the type.
/// Supports both nullable and option types.
[<Extension>]
static member Field<'T> (this:IDataRecord, ordinal:int) =
this.Field<'T>(ordinal)
在命令行应用程序中处理参数：
//We assume that the actual meat is already defined in function
// DoStuff (string -> string -> string -> unit)
let defaultOutOption = "N"
let defaultUsageOption = "Y"
let usage =
"Scans a folder for and outputs results.\n" +
"Usage:\n\t MyApplication.exe FolderPath [IncludeSubfolders (Y/N) : default=" +
defaultUsageOption + "] [OutputToFile (Y/N): default=" + defaultOutOption + "]"
let HandlArgs arr =
match arr with
| [|d;u;o|] -> DoStuff d u o
| [|d;u|] -> DoStuff d u defaultOutOption
| [|d|] -> DoStuff d defaultUsageOption defaultOutOption
| _ ->
printf "%s" usage
Console.ReadLine() |> ignore
[<EntryPoint>]
let main (args : string array) =
args |> HandlArgs
0
（我对受Robert Pickering启发的这项技术有一个模糊的记忆，但现在找不到参考资料）
创建 XElement
没什么了不起的，但我一直被 XNames 的隐式转换所吸引：
#r "System.Xml.Linq.dll"
open System.Xml.Linq
//No! ("type string not compatible with XName")
//let el = new XElement("MyElement", "text")
//better
let xn s = XName.op_Implicit s
let el = new XElement(xn "MyElement", "text")
//or even
let xEl s o = new XElement(xn s, o)
let el = xEl "MyElement" "text"
成对和成对
我总是希望 Seq.pairwise 给我 [(1,2);(3;4)] 而不是 [(1,2);(2,3);(3,4)]。鉴于 List 中都不存在，并且我需要两者，这是供将来参考的代码。我认为它们是尾递归的。
//converts to 'windowed tuples' ([1;2;3;4;5] -> [(1,2);(2,3);(3,4);(4,5)])
let pairwise lst =
let rec loop prev rem acc =
match rem with
| hd::tl -> loop hd tl ((prev,hd)::acc)
| _ -> List.rev acc
loop (List.head lst) (List.tail lst) []
//converts to 'paged tuples' ([1;2;3;4;5;6] -> [(1,2);(3,4);(5,6)])
let pairs lst =
let rec loop rem acc =
match rem with
| l::r::tl -> loop tl ((l,r)::acc)
| l::[] -> failwith "odd-numbered list"
| _ -> List.rev acc
loop lst []
一个方便的缓存功能，可以保存max (key,reader(key))在字典中并使用 aSortedList来跟踪 MRU 键
let Cache (reader: 'key -> 'value) max =
let cache = new Dictionary<'key,LinkedListNode<'key * 'value>>()
let keys = new LinkedList<'key * 'value>()
fun (key : 'key) -> (
let found, value = cache.TryGetValue key
match found with
|true ->
keys.Remove value
keys.AddFirst value |> ignore
(snd value.Value)
|false ->
let newValue = key,reader key
let node = keys.AddFirst newValue
cache.[key] <- node
if (keys.Count > max) then
let lastNode = keys.Last
cache.Remove (fst lastNode.Value) |> ignore
keys.RemoveLast() |> ignore
(snd newValue))
日期范围
fromDate和之间的简单但有用的日期列表toDate
let getDateRange fromDate toDate =
let rec dates (fromDate:System.DateTime) (toDate:System.DateTime) =
seq {
if fromDate <= toDate then
yield fromDate
yield! dates (fromDate.AddDays(1.0)) toDate
}
dates fromDate toDate
|> List.ofSeq
将代码切换到 sql
比这个列表中的大多数都微不足道，但仍然很方便：
在开发过程中，我总是将 sql 进出代码以将其移动到 sql 环境中。例子：
let sql = "select a,b,c "
+ "from table "
+ "where a = 1"
需要“剥离”到：
select a,b,c
from table
where a = 1
保持格式。去掉 sql 编辑器的代码符号，然后在我弄好 sql 时再手动放回去，这很痛苦。这两个函数来回切换 sql 从代码到剥离：
// reads the file with the code quoted sql, strips code symbols, dumps to FSI
let stripForSql fileName =
File.ReadAllText(fileName)
|> (fun s -> Regex.Replace(s, "\+(\s*)\"", ""))
|> (fun s -> s.Replace("\"", ""))
|> (fun s -> Regex.Replace(s, ";$", "")) // end of line semicolons
|> (fun s -> Regex.Replace(s, "//.+", "")) // get rid of any comments
|> (fun s -> printfn "%s" s)
然后当您准备好将其放回代码源文件时：
let prepFromSql fileName =
File.ReadAllText(fileName)
|> (fun s -> Regex.Replace(s, @"\r\n", " \"\r\n+\"")) // matches newline
|> (fun s -> Regex.Replace(s, @"\A", " \""))
|> (fun s -> Regex.Replace(s, @"\z", " \""))
|> (fun s -> printfn "%s" s)
我很想摆脱输入文件，但甚至无法开始了解如何做到这一点。任何人？
编辑：
我想出了如何通过添加 Windows 窗体对话框输入/输出来消除对这些功能的文件要求。代码太多要显示，但是对于那些想做这样的事情的人，这就是我解决它的方法。
天真的 CSV 阅读器（即，不会处理任何讨厌的事情）
（在这里使用其他答案中的 filereadlines 和 List.transpose ）
///Given a file path, returns a List of row lists
let ReadCSV =
filereadlines
>> Array.map ( fun line -> line.Split([|',';';'|]) |> List.ofArray )
>> Array.toList
///takes list of col ids and list of rows,
/// returns array of columns (in requested order)
let GetColumns cols rows =
//Create filter
let pick cols (row:list<'a>) = List.map (fun i -> row.[i]) cols
rows
|> transpose //change list of rows to list of columns
|> pick cols //pick out the columns we want
|> Array.ofList //an array output is easier to index for user
例子
"C:\MySampleCSV"
|> ReadCSV
|> List.tail //skip header line
|> GetColumns [0;3;1] //reorder columns as well, if needs be.
帕斯卡三角（嘿，有人可能会觉得它有用）
所以我们想创建一个像这样的东西：
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
很简单：
let rec next = function
| [] -> []
| x::y::xs -> (x + y)::next (y::xs)
| x::xs -> x::next xs
let pascal n =
seq { 1 .. n }
|> List.scan (fun acc _ -> next (0::acc) ) [1]
该next函数返回一个新列表，其中每个 item[i] = item[i] + item[i + 1]。
这是 fsi 中的输出：
> pascal 10 |> Seq.iter (printfn "%A");;
[1]
[1; 1]
[1; 2; 1]
[1; 3; 3; 1]
[1; 4; 6; 4; 1]
[1; 5; 10; 10; 5; 1]
[1; 6; 15; 20; 15; 6; 1]
[1; 7; 21; 35; 35; 21; 7; 1]
[1; 8; 28; 56; 70; 56; 28; 8; 1]
[1; 9; 36; 84; 126; 126; 84; 36; 9; 1]
[1; 10; 45; 120; 210; 252; 210; 120; 45; 10; 1]
对于喜欢冒险的人，这里有一个尾递归版本：
let rec next2 cont = function
| [] -> cont []
| x::y::xs -> next2 (fun l -> cont <| (x + y)::l ) <| y::xs
| x::xs -> next2 (fun l -> cont <| x::l ) <| xs
let pascal2 n =
set { 1 .. n }
|> Seq.scan (fun acc _ -> next2 id <| 0::acc)) [1]
展平列表
如果你有这样的事情：
let listList = [[1;2;3;];[4;5;6]]
并希望将其“展平”为单个列表，因此结果如下：
[1;2;3;4;5;6]
可以这样做：
let flatten (l: 'a list list) =
seq {
yield List.head (List.head l)
for a in l do yield! (Seq.skip 1 a)
}
|> List.ofSeq
浮点数的列表推导
这[23.0 .. 1.0 .. 40.0]被标记为已弃用，有几个版本支持。
但显然，这有效：
let dl = 9.5 / 11.
let min = 21.5 + dl
let max = 40.5 - dl
let a = [ for z in min .. dl .. max -> z ]
let b = a.Length
（顺便说一句，那里有一个浮点陷阱。在fssnip发现- F# 片段的另一个地方）
将记录设置为空
type Foo = { x : int }
let inline retype (x:'a) : 'b = (# "" x : 'b #)
let f : Foo = retype null
平行图
let pmap f s =
seq { for a in s -> async { return f s } }
|> Async.Parallel
|> Async.Run