Handy F# snippets

Question

There are already two questions about F#/functional snippets.

However what I'm looking for here are useful snippets, little 'helper' functions that are reusable. Or obscure but nifty patterns that you can never quite remember.

Something like:

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} 

I'd like to make this a kind of handy reference page. As such there will be no right answer, but hopefully lots of good ones.

EDIT Tomas Petricek has created a site specifically for F# snippets http://fssnip.net/.

Answer 1

Perl style regex matching

let (=~) input pattern =
    System.Text.RegularExpressions.Regex.IsMatch(input, pattern)

It lets you match text using let test = "monkey" =~ "monk.+" notation.

Answer 2

Infix Operator

I got this from http://sandersn.com/blog//index.php/2009/10/22/infix-function-trick-for-f go to that page for more details.

If you know Haskell, you might find yourself missing infix sugar in F#:

// 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" 

While F# doesn't have a true 'infix' operator, the same thing can be accomplished almost as elegantly via a pipeline and a 'backpipeline' (who knew of such a thing??)

// F# 'infix' trick via pipelines
"kevin" |> startsWith <| "K"

Answer 3

Multi-Line Strings

This is pretty trivial, but it seems to be a feature of F# strings that is not widely known.

let sql = "select a,b,c \
           from table \
           where a = 1"

This produces:

val sql : string = "select a,b,c from table where a = 1"

When the F# compiler sees a back-slash followed by a carriage return inside a string literal, it will remove everything from the back-slash to the first non-space character on the next line. This allows you to have multi-line string literals that line up, without using a bunch of string concatenation.

Answer 4

Generic memoization, courtesy of the man himself

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

Using this, you could do a cached reader like so:

let cachedReader = memoize reader

Answer 5

Simple read-write to text files

These are trivial, but make file access pipeable:

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)

So

let replace f (r:string) (s:string) = s.Replace(f, r)

"C:\\Test.txt" |>
    fileread |>
    replace "teh" "the" |>
    filewrite "C:\\Test.txt"

And combining that with the visitor quoted in the question:

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)

Update Slight improvement if you want to be able to read 'locked' files (e.g. csv files which are already open in Excel...):

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  

Answer 6

For performance intensive stuff where you need to check for null

let inline isNull o = System.Object.ReferenceEquals(o, null)
if isNull o then ... else ...

Is about 20x faster then

if o = null then ... else ...

Answer 7

Active Patterns, aka "Banana Splits", are a very handy construct that let one match against multiple regular expression patterns. This is much like AWK, but without the high performance of DFA's because the patterns are matched in sequence until one succeeds.

#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

Some examples of use:

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

Answer 8

'Unitize' a function which doesn't handle units Using the FloatWithMeasure function 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))

Example:

[<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>

---

OLD version:

let unitize (f:float -> float) (v:float<'u>) =
  let unit = box 1. :?> float<'u>
  unit * (f (v/unit))

Kudos to kvb

Answer 9

Maybe monad

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()

Here's a brief intro to monads for the uninitiated.

Answer 10

Option-coalescing operators

I wanted a version of the defaultArg function that had a syntax closer to the C# null-coalescing operator, ??. This lets me get the value from an Option while providing a default value, using a very concise syntax.

/// 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()

Answer 11

Scale/Ratio function builder

Again, trivial, but handy.

//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))

Example:

[<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>

Answer 12

F# Map <-> C# Dictionary

(I know, I know, System.Collections.Generic.Dictionary isn't really a 'C#' dictionary)

C# to F#

(dic :> seq<_>)                        //cast to seq of KeyValuePair
    |> Seq.map (|KeyValue|)            //convert KeyValuePairs to tuples
    |> Map.ofSeq                       //convert to Map

(From Brian, here, with improvement proposed by Mauricio in comment below. (|KeyValue|) is an active pattern for matching KeyValuePair - from FSharp.Core - equivalent to (fun kvp -> kvp.Key, kvp.Value))

Interesting alternative

To get all of the immutable goodness, but with the O(1) lookup speed of Dictionary, you can use the dict operator, which returns an immutable IDictionary (see this question).

I currently can't see a way to directly convert a Dictionary using this method, other than

(dic :> seq<_>)                        //cast to seq of KeyValuePair
    |> (fun kvp -> kvp.Key, kvp.Value) //convert KeyValuePairs to tuples
    |> dict                            //convert to immutable IDictionary

F# to C#

let dic = Dictionary()
map |> Map.iter (fun k t -> dic.Add(k, t))
dic

What is weird here is that FSI will report the type as (for example):

val it : Dictionary<string,int> = dict [("a",1);("b",2)]

but if you feed dict [("a",1);("b",2)] back in, FSI reports

IDictionary<string,int> = seq[[a,1] {Key = "a"; Value = 1; } ...

Answer 13

Transposing a list (seen on Jomo Fisher's blog)

///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]]

And here is a tail-recursive version which (from my sketchy profiling) is mildly slower, but has the advantage of not throwing a stack overflow when the inner lists are longer than 10000 elements (on my machine):

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

If I was clever, I'd try and parallelise it with async...

Answer 14

Tree-sort / Flatten a tree into a list

I have the following binary tree:

             ___ 77 _
            /        \
   ______ 47 __       99
  /            \
21 _          54
    \        /  \
      43    53  74
     /
    39
   /
  32

Which is represented as follows:

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))

A straightforward method to flatten the tree is:

let rec flatten = function
    | Nil -> []
    | Node(l, a, r) -> flatten l @ a::flatten r

This isn't tail-recursive, and I believe the @ operator causes it to be O(n log n) or O(n^2) with unbalanced binary trees. With a little tweaking, I came up with this tail-recursive O(n) version:

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

Here's the output in fsi:

> flatten2 myTree;;
val it : int list = [21; 32; 39; 43; 47; 53; 54; 74; 77; 99]

Answer 15

LINQ-to-XML helpers

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)

Answer 16

OK, this has nothing to do with snippets, but I keep forgetting this:

If you are in the interactive window, you hit F7 to jump back to the code window (without deselecting the code which you just ran...)

Going from code window to F# window (and also to open the F# window) is Ctrl Alt F

(unless CodeRush has stolen your bindings...)

Answer 17

Weighted sum of arrays

Calculating a weighted [n-array] sum of a [k-array of n-arrays] of numbers, based on a [k-array] of weights

(Copied from this question, and kvb's answer)

Given these arrays

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|] |]

We want a weighted sum (by column), given that both dimensions of the arrays can be variable.

Array.map2 (fun w -> Array.map ((*) w)) weights arrs 
|> Array.reduce (Array.map2 (+))

First line: Partial application of the first Array.map2 function to weights yields a new function (Array.map ((*) weight) which is applied (for each weight) to each array in arr.

Second line: Array.reduce is like fold, except it starts on the second value and uses the first as the initial 'state'. In this case each value is a 'line' of our array of arrays. So applying an Array.map2 (+) on the first two lines means that we sum the first two arrays, which leaves us with a new array, which we then (Array.reduce) sum again onto the next (in this case last) array.

Result:

[|0.060123; 0.069444; 0.07296; 1.5510666; 215.40356|]

Answer 18

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))

Answer 19

DataSetExtensions for F#, DataReaders

System.Data.DataSetExtensions.dll adds the ability to treat a DataTable as an IEnumerable<DataRow> as well as unboxing the values of individual cells in a way that gracefully handles DBNull by supporting System.Nullable. For example, in C# we can get the value of an integer column that contains nulls, and specify that DBNull should default to zero with a very concise syntax:

var total = myDataTable.AsEnumerable()
                       .Select(row => row.Field<int?>("MyColumn") ?? 0)
                       .Sum();

There are two areas where DataSetExtensions are lacking, however. First, it doesn't support IDataReader and second, it doesn't support the F# option type. The following code does both - it allows an IDataReader to be treated as a seq<IDataRecord>, and it can unbox values from either a reader or a dataset, with support for F# options or System.Nullable. Combined with the option-coalescing operator in another answer, this allows for code such as the following when working with a DataReader:

let total =
    myReader.AsSeq
    |> Seq.map (fun row -> row.Field<int option>("MyColumn") |? 0)
    |> Seq.sum

Perhaps a more idiomatic F# way of ignoring database nulls would be...

let total =
    myReader.AsSeq
    |> Seq.choose (fun row -> row.Field<int option>("MyColumn"))
    |> Seq.sum

Further, the extension methods defined below are usable from both F# and from 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)

Answer 20

Handling arguments in a command line application:

//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

(I had a vague memory of this technique being inspired by Robert Pickering, but can't find a reference now)

Answer 21

Creating XElements

Nothing amazing, but I keep getting caught out by the implicit conversion of XNames:

#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"

Answer 22

Pairwise and pairs

I always expect Seq.pairwise to give me [(1,2);(3;4)] and not [(1,2);(2,3);(3,4)]. Given that neither exist in List, and that I needed both, here's the code for future reference. I think they're tail recursive.

//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 []

Answer 23

A handy cache function that keeps up to max (key,reader(key)) in a dictionary and use a SortedList to track the MRU keys

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))

Answer 24

Naive CSV reader (i.e., won't handle anything nasty)

(Using filereadlines and List.transpose from other answers here)

///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

Example

"C:\MySampleCSV"
   |> ReadCSV
   |> List.tail //skip header line
   |> GetColumns [0;3;1]  //reorder columns as well, if needs be.

Answer 25

Pascal's Triangle (hey, someone might find it useful)

So we want to create a something like this:

       1
      1 1
     1 2 1
    1 3 3 1
   1 4 6 4 1

Easy enough:

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]

The next function returns a new list where each item[i] = item[i] + item[i + 1].

Here's the output in fsi:

> 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]

For the adventurous, here's a tail-recursive version:

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]

Answer 26

Date Range

simple but useful list of dates between fromDate and 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

Answer 27

toggle code to sql

More trivial than most on this list, but handy nonetheless:

I'm always taking sql in and out of code to move it to a sql environment during development. Example:

let sql = "select a,b,c "
    + "from table "
    + "where a = 1"

needs to be 'stripped' to:

select a,b,c
from table
where a = 1

keeping the formatting. It's a pain to strip out the code symbols for the sql editor, then put them back again by hand when I've got the sql worked out. These two functions toggle the sql back and forth from code to stripped:

// 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)

then when you are ready to put it back into your code source file:

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)

I'd love to get rid of the input file but can't even begin to grok how to make that happen. anyone?

edit:

I figured out how to eliminate the requirement of a file for these functions by adding a windows forms dialog input/output. Too much code to show, but for those who would like to do such a thing, that's how I solved it.

Answer 28

Flatten a List

if you have something like this:

let listList = [[1;2;3;];[4;5;6]] 

and want to 'flatten' it down to a singe list so the result is like this:

[1;2;3;4;5;6]

it can be done thusly:

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

Answer 29

List comprehensions for float

This [23.0 .. 1.0 .. 40.0] was marked as deprecated a few versions backed.

But apparently, this works:

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

(BTW, there's a floating point gotcha in there. Discovered at fssnip - the other place for F# snippets)

Answer 30

Parallel map

let pmap f s =
    seq { for a in s -> async { return f s } }
    |> Async.Parallel
    |> Async.Run

Answer 31

Setting a record to null

type Foo = { x : int }
let inline retype (x:'a) : 'b = (# "" x : 'b #)
let f : Foo = retype null