Getting Started with Twitch | Twitch Thrashing Code Stream

I’d been meaning to get started for some time. I even tweeted, just trying to get further insight into what and why people watch Twitch streams and of course why and who produces their own Twitch streams.

With that I set out to figure out how to get the right tooling setup for Twitch streaming on Linux and MacOS. Here’s what I managed to dig up.

First things first, and somewhat obviously, go create a Twitch account. The sign up link is up in the top right corner. Continue reading “Getting Started with Twitch | Twitch Thrashing Code Stream”

Cassandra / DataStax Enterprise 6 Clusters: Marketplace Options

As I have stepped full speed into work and research at DataStax there were a few things I needed as soon as I could possibly get them put together. Before even diving into development, use case examples, or reference application development I needed to have some clusters built up. The Docker image is great for some simple local development, but beyond that I wanted to have some live 3+ node clusters to work with. The specific deployed and configured use cases I had included:

  1. I wanted to have a DataStax Enterprise 6 Cassandra Cluster up and running ASAP. A cluster that would be long lived that I could developer sample applications against, use for testing purposes, and generally develop against from a Cassandra and DSE purpose.
  2. I wanted to have an easy to use cluster setup for Cassandra – just the OSS deployment – possibly coded and configured for deployment with Terraform and related scripts necessary to get a 3 node cluster up and running in Google Cloud Platform, Azure, or AWS.
  3. I wanted a DataStax Enterprise 6 enabled deployment, that would showcase some of the excellent tooling DataStax has built around the database itself.

I immediately set out to build solutions for these three requirements.

The first cluster system I decided to aim for was figuring out a way to get some reasonably priced hardware to actually build a physical cluster. Something that would make it absurdly easy to just have something to work with anytime I want without incurring additional expenses. Kind of the ultimate local development environment. With that I began scouring the interwebs and checking out where or how I could get some boxes to build this cluster with. I also reached out to a few people to see if I could be gifted some boxes from Dell or another manufacturer.

I lucked out and found some cheap boxes someone was willing to send over my way for almost nothing. But in the meantime since shipping will take a week or two. I began scouring the easy to get started options on AWS, Google Cloud Platform, and Azure. Continue reading “Cassandra / DataStax Enterprise 6 Clusters: Marketplace Options”

_____101 |> F# Coding Ecosystem: Paket && Atom w/ Paket

One extremely useful tool to use with F# is Paket. Paket is a package manager that provides a super clean way to manage your dependencies. Paket can handle everything from Nuget dependencies to git or file dependencies. It really opens up your project capabilities to easily pull in and handle dependencies, whereever they are located.

I cloned the Paket Project first, since I would like to have the very latest and help out if anything came up. For more information on Paket check out the about page.

git clone

I built that project with the respective ./ script and all went well.


NOTE – Get That Command Line Action

One thing I didn’t notice immediately in the docs (I’m putting in a PR right after this blog entry) was anyway to actually get Paket setup for the command line. On bash, Windows, or whatever, it seemed a pretty fundamental missing piece so I’m going to doc that right here but also submit a PR based on the issue I added here). It could be I just missed it, but either way, here’s the next step that will get you setup the rest of the way.


Yeah, that’s all it was. Kind of silly eh? Maybe that’s why it isn’t documented that I could see? After the installation script is run, just execute paket and you’ll get the list of the various commands, as shown below.

$ paket
Paket version
Command was:
available commands:

	add: Adds a new package to your paket.dependencies file.
	config: Allows to store global configuration values like NuGet credentials.
	convert-from-nuget: Converts from using NuGet to Paket.
	find-refs: Finds all project files that have the given NuGet packages installed.
	init: Creates an empty paket.dependencies file in the working directory.
	auto-restore: Enables or disables automatic Package Restore in Visual Studio during the build process.
	install: Download the dependencies specified by the paket.dependencies or paket.lock file into the `packages/` directory and update projects.
	outdated: Lists all dependencies that have newer versions available.
	remove: Removes a package from your paket.dependencies file and all paket.references files.
	restore: Download the dependencies specified by the paket.lock file into the `packages/` directory.
	simplify: Simplifies your paket.dependencies file by removing transitive dependencies.
	update: Update one or all dependencies to their latest version and update projects.
	find-packages: EXPERIMENTAL: Allows to search for packages.
	find-package-versions: EXPERIMENTAL: Allows to search for package versions.
	show-installed-packages: EXPERIMENTAL: Shows all installed top-level packages.
	pack: Packs all paket.template files within this repository
	push: Pushes all `.nupkg` files from the given directory.

	--help [-h|/h|/help|/?]: display this list of options.

Paket Elsewhere && Atom

If you’re interested in Paket with Visual Studio I’ll let you dig into that on your own. Some resources are Paket Visual Studio on Github and Paket for Visual Studio. What I was curious though was Paket integration with either Atom or Visual Studio Code.

Krzysztof Cieślak (@k_cieslak) and Stephen Forkmann (@sforkmann) maintain the Paket.Atom Project and Krzysztof Cieślak also handles the atom-fsharp project for Atom. Watch this gif for some of the awesome goodies that Atom gets with the Paket.Atom Plugin.

Click for fullsize image of the gif.

Click for fullsize image of the gif.

Getting Started and Adding Dependencies

I’m hacking along and want to add some libraries, how do I do that with Paket? Let’s take a look. This is actually super easy, and doesn’t make the project dependentant on peripheral tooling like Visual Studio when using Paket.

The first thing to do, is inside the directory or project where I need the dependency I’ll intialize the it for paket.

paket init

The next step is to add the dependency or dependencies that I’ll need. I’ll add a Nuget package that I’ll need shortly. The first package I want to grab for this project is FsUnit, a testing framework project managed and maintained by Dan Mohl @dmohl and Sergey Tihon @sergey_tihon.

paket add nuget FsUnit

When executing this dependency addition the results displayed show what other dependencies were installed and which versions were pegged for this particular dependency.

✔ ~/Codez/sharpPaketsExample
15:33 $ paket add nuget FsUnit
Paket version
Adding FsUnit to /Users/halla/Codez/sharpPaketsExample/paket.dependencies
Resolving packages:
 - FsUnit 1.3.1
 - NUnit 2.6.4
Locked version resolution written to /Users/halla/Codez/sharpPaketsExample/paket.lock
Dependencies files saved to /Users/halla/Codez/sharpPaketsExample/paket.dependencies
Downloading FsUnit 1.3.1 to /Users/halla/.local/share/NuGet/Cache/FsUnit.1.3.1.nupkg
NUnit 2.6.4 unzipped to /Users/halla/Codez/sharpPaketsExample/packages/NUnit
FsUnit 1.3.1 unzipped to /Users/halla/Codez/sharpPaketsExample/packages/FsUnit
3 seconds - ready.

I took a look in the packet.dependencies and packet.lock file to see what were added for me with the paket add nuget command. The packet.dependencies file looked like this now.


nuget FsUnit

The packet.lock file looked like this.

    FsUnit (1.3.1)
      NUnit (2.6.4)
    NUnit (2.6.4)

There are a few more dependencies that I want, so I went to work adding those. First of this batch that I added was FAKE (more on this in a subsequent blog entry), which is a build tool based off of RAKE.

paket add nuget FAKE

Next up was FsCheck.

paket add nuget FsCheck

The paket.dependencies file now had the following content.


nuget FAKE
nuget FsCheck
nuget FsUnit

The paket.lock file had the following items added.

    FAKE (4.1.4)
    FsCheck (2.0.7)
      FSharp.Core (>=
    FSharp.Core (
    FsUnit (1.3.1)
      NUnit (2.6.4)
    NUnit (2.6.4)

Well, that got me started. The code repository at this state is located on this branch here of the sharpSystemExamples repository. So on to some coding and the next topic. Keep reading, subsribe, or hit me up on twitter @adron.


Docker Tips n’ Tricks for Devs – #0001 – 3 Second to Postgresql

The easiest implementation of a docker container with Postgresql that I’ve found recently allows the following commands to pull and run a Postgresql server for you.

docker pull postgres:latest
docker run -p 5432:5432 postgres

Then you can just connect to the Postgresql Server by opening up pgadmin with the following connection information:

  • Host: localhost
  • Port: 5432
  • Maintenance DB: postgres
  • Username: postgres
  • Password:

With that information you’ll be able to connect and use this as a development database that only takes about 3 seconds to launch whenever you need it.

______10 |> F# – Moar Thinking Functionally (Notes)

More notes on the “Thinking Functionally” series. Previous notes are @ “_______1 |> F# – Getting Started, Thinking Functionally“.

#6 Partial Application

Breaking down functions into single parameter functions is the mathematically correct way of doing it, but that is not the only reason it is done — it also leads to a very powerful technique called partial function application.

For example:

let add42 = (+) 42 // partial application
add42 1
add42 3

[1;2;3] |> add42

let twoIsLessThan = (<) 2 // partial application twoIsLessThan 1 twoIsLessThan 3 // filter each element with the twoIsLessThan function [1;2;3] |> List.filter twoIsLessThan

let printer = printfn "printing param=%i"

[1;2;3] |> List.iter printer

Each case a partially applied function above it can then be reused in multiple contexts. It can also fix function parameters.

let add1 = (+) 1
let add1ToEach = add1   // fix the "add1" function

add1ToEach [1;2;3;4]

let filterEvens =
   List.filter (fun i -> i%2 = 0) // fix the filter function

filterEvens [1;2;3;4]

Then the following shows plug in behavior that is transparent.

let adderWithPluggableLogger logger x y =
    logger "x" x
    logger "y" y
    let result = x + y
    logger "x+y"  result

let consoleLogger argName argValue =
    printfn "%s=%A" argName argValue 

let addWithConsoleLogger = adderWithPluggableLogger consoleLogger
addWithConsoleLogger  1 2
addWithConsoleLogger  42 99

let popupLogger argName argValue =
    let message = sprintf "%s=%A" argName argValue
      |> ignore

let addWithPopupLogger  = adderWithPluggableLogger popupLogger
addWithPopupLogger  1 2
addWithPopupLogger  42 99

Designing Functions for Partial Application

Sample calls to the list library:    (fun i -> i+1) [0;1;2;3]
List.filter (fun i -> i>1) [0;1;2;3]
List.sortBy (fun i -> -i ) [0;1;2;3]

Here are the same examples using partial application:

let eachAdd1 = (fun i -> i+1)
eachAdd1 [0;1;2;3]

let excludeOneOrLess = List.filter (fun i -> i>1)
excludeOneOrLess [0;1;2;3]

let sortDesc = List.sortBy (fun i -> -i)
sortDesc [0;1;2;3]

Commonly accepted guidelines to multi-parameter function design.

  1. Put earlier: parameters ore likely to be static. The parameters that are most likely to be “fixed” with partial application should be first.
  2. Put last: the data structure or collection (or most varying argument). Makes it easier to pipe a structure or collection from function to function. Like:
    let result =
       |> (fun i -> i+1)
       |> List.filter (fun i -> i>5)
  3. For well-known operations such as “subtract”, put in the expected order.

Wrapping BCL Function for Partial Application

Since the data parameter is generally last versus most BCL calls that have the data parameter first, it’s good to wrap the BCL.

let replace oldStr newStr (s:string) =
  s.Replace(oldValue=oldStr, newValue=newStr)

let startsWith lookFor (s:string) =

Then pipes can be used with the BCL call in the expected way.

let result =
     |> replace "h" "j"
     |> startsWith "j"

["the"; "quick"; "brown"; "fox"]
     |> List.filter (startsWith "f")

…or we can use function composition.

let compositeOp = replace "h" "j" >> startsWith "j"
let result = compositeOp "hello"

Understanding the Pipe Function

The pipe function is defined as:

let (|>) x f = f x

It allows us to put the function argument in front of the function instead of after.

let doSomething x y z = x+y+z
doSomething 1 2 3

If the function has multiple parameters, then it appears that the input is the final parameter. Actually what is happening is that the function is partially applied, returning a function that has a single parameter: the input.

let doSomething x y  =
   let intermediateFn z = x+y+z
   intermediateFn        // return intermediateFn

let doSomethingPartial = doSomething 1 2
doSomethingPartial 3
3 |> doSomethingPartial

#7 Function Associativity and Composition

Function Associativity


let F x y z = x y z

…means this…

let F x y z = (x y) z

Also three equivalent forms.

let F x y z = x (y z)
let F x y z = y z |> x
let F x y z = x <| y z

Function Composition

Here’s an example

let f (x:int) = float x * 3.0  // f is int->float
let g (x:float) = x > 4.0      // g is float->bool

We can create a new function h that takes the output of “f” and uses it as the input for “g”.

let h (x:int) =
    let y = f(x)
    g(y)                   // return output of g

A much more compact way is this:

let h (x:int) = g ( f(x) ) // h is int->bool

h 1
h 2

These are notes, to read more check out the Function Composition.

______11 |> F# – Some Hackery – A String Calculator Kata

Now for some F# hacking. The first thing I did was actually go through a Code Kata, which I’ll present here.

The first step I took was to get a project started. For that I used the ProjectScaffold to build a clean project via bash.

First cloned…

git clone sharpKataStringCalc

…then I navigated into the directory and executed the script…

cd sharpKataStringCalc/

…then I got prompted for some input.


# Project Scaffold Init Script
# Please answer a few questions and we will generate
# two files:
# build.fsx               This will be your build script
# docs/tools/generate.fsx This script will generate your
#                         documentation
# NOTE: Aside from the Project Name, you may leave any
# of these blank, but you will need to change the defaults
# in the generated scripts.


Project Name (used for solution/project files): sharpKataStringCalc
Summary (a short description): A code kata for the string calculator exercise.
Description (longer description used by NuGet): The code kata, kicked off my Roy Osherove, this is my iteration of it (at least my first iteration of it).
Author: Adron Hall
Tags (separated by spaces): fsharp f# code kata stringcalculator
Github User or Organization: adron
Github Project Name (leave blank to use Project Name):

Once I hit enter after entering the information I’ve gotten more than a few of these broken builds.

Time Elapsed 00:00:00.1609190
Running build failed.
Building /Users/adronhall/Coderz/sharpKataStringCalc/sharpKataStringCalc.sln failed with exitcode 1.

Build Time Report
Target         Duration
------         --------
Clean          00:00:00.0019508
AssemblyInfo   00:00:00.0107624
Total:         00:00:00.6460652
Status:        Failure
  1) Building /Users/adronhall/Coderz/sharpKataStringCalc/sharpKataStringCalc.sln failed with exitcode 1.
  2) : /Users/adronhall/Coderz/sharpKataStringCalc/src/sharpKataStringCalc/sharpKataStringCalc.fsproj(0,0): Target named 'Rebuild' not found in the project.
  3) : /Users/adronhall/Coderz/sharpKataStringCalc/tests/sharpKataStringCalc.Tests/sharpKataStringCalc.Tests.fsproj(0,0): /Users/adronhall/Coderz/sharpKataStringCalc/tests/sharpKataStringCalc.Tests/sharpKataStringCalc.Tests.fsproj: The required attribute "Project" in Import is empty

This problem I was able to solve once, based on what I did in a previous blog entry “That Non-Windows Scaffolding for OS-X and Linux |> I Broke It! But…“. Which seemed odd that I fixed it previously. To help with the build I actually opened it up in Xamarin Studio. Now, one of the problems with doing this, is that it’s only available on Windows & OS-X. I’m however interested in using this stuff on Linux too, but that’s looking a bit more difficult the more I work with the toolchain unfortunately.

After working through the issue I found that on one OS-X box I’d installed Mono via make and F# via make and that messes things up. Do one or the other and you should be ok. So on my other two OS-X boxes (I’ve a personal retina and a work retina) the build worked flawlessly, and when it works flawlessly it looks like this toward the end of the build execution.

Finished Target: GenerateReferenceDocs
Starting Target: GenerateDocs (==> GenerateReferenceDocs, GenerateReferenceDocs)
Finished Target: GenerateDocs
Starting Target: All (==> GenerateDocs)
Finished Target: All

Build Time Report
Target                  Duration
------                  --------
Clean                   00:00:00.0035253
AssemblyInfo            00:00:00.0103142
Build                   00:00:04.9369669
CopyBinaries            00:00:00.0052210
RunTests                00:00:00.6568475
CleanDocs               00:00:00.0025772
GenerateHelp            00:00:08.6989318
GenerateReferenceDocs   00:00:11.7627584
GenerateDocs            00:00:00.0003409
All                     00:00:00.0000324
Total:                  00:00:26.1162623
Status:                 Ok

I’ve gotten this to work on OS-X and Windows just fine using the straight up ProjectScaffold and the ./ So all is good, I’m going to move forward with writing the kata based on that and loop back around to straighten out the Linux issues.

To run the tests, execute the following script after creating the project scaffold.

./ RunTests

First off, what are the ideas behind the string calculator kata? Well here’s how Roy Osherove lays it out this particular code kata.

Before you start:

  • Try not to read ahead.
  • Do one task at a time. The trick is to learn to work incrementally.
  • Make sure you only test for correct inputs. there is no need to test for invalid inputs for this kata.

String Calculator

  1. Create a simple String calculator with a method int Add(string numbers)
    1. The method can take 0, 1 or 2 numbers, and will return their sum (for an empty string it will return 0) for example “” or “1” or “1 2”
    2. Start with the simplest test case of an empty string and move to 1 and two numbers
    3. Remember to solve things as simply as possible so that you force yourself to write tests you did not think about
    4. Remember to refactor after each passing test
  2. Allow the Add method to handle an unknown amount of numbers.
  3. Allow the Add method to handle new lines between numbers (instead of an empty space).
    1. the following input is ok: “1\n2 3” (will equal 6)
    2. the following input is NOT ok: “1 \n” (not need to prove it – just clarifying)
  4. Support different delimiters
    1. to change a delimiter, the beginning of the string will contain a separate line that looks like this: “//[delimiter]\n[numbers…]” for example “//;\n1;2” should return three where the default delimiter is ‘;’ .
    2. the first line is optional. all existing scenarios should still be supported
  5. Calling Add with a negative number will throw an exception “negatives not allowed” – and the negative that was passed.if there are multiple negatives, show all of them in the exception message
  6. Numbers bigger than 1000 should be ignored, so adding 2 + 1001 = 2
  7. Delimiters can be of any length with the following format: “//[delimiter]\n” for example: “//[***]\n1***2***3” should return 6
  8. Allow multiple delimiters like this: “//[delim1][delim2]\n” for example “//[*][%]\n1*2%3” should return 6.
  9. Make sure you can also handle multiple delimiters with length longer than one char.

Ok, so now that we’re clear on the string calculator, I’m going to dig into knocking out the first item, “Create a simple string calculator with a method int Add (string numbers)”

But first, in TDD fashion let’s write the test and make it fail first. I changed the code in the Tests.fs file in the tests directory and tests project to read as follows.

module sharpKataStringCalc.Tests

open System
open sharpKataStringCalc
open NUnit.Framework

type CalculatorTests() =
  member x.add_empty_string() =
    let calculator = Calculator()
    let result = calculator.Add ""
    Assert.That(result, Is.EqualTo 0)

That gets us a failing test, since we don’t even have any implementation yet. So now I’ll add the first part of the implementation code. First I created a Calculator.fs file and deleted the other file that ProjectScaffold put in there in the first place.

namespace sharpKataStringCalc

open System

type Calculator() = 
  member x.Add express = 

Ok, that gives me a passing test for the first phase of all this. Now since I’m a total F# newb still I’ve got to kind of dig around and read documentation while I’m working through this. So I’m taking a couple of hours while Roy’s suggestion is to use 30 minutes to do this kata. But I figured it is a good way to force myself to learn the syntax and start getting into an F# refactoring practice.

The first thing I started to do was write a test where I set the Calculator() again that looked something like this. I didn’t like that so I tried to pull it out of the test.

  [<TestCase("1", Result = 1)>]
  member x.Add_single_number_returns_that_number expression =
    let calculator = Calculator()
    calculator.Add expression

I ended up with something like this then.

let calculator = Calculator()

type CalculatorTests() =
  member x.add_empty_string() =
    let result = calculator.Add ""
    Assert.That(result, Is.EqualTo 0)

  [<TestCase("1", Result = 1)>]
  member x.Add_single_number_returns_that_number expression =
    calculator.Add expression

After adding that code with that little refactor I ran it, red light fail, so I then moved on to implementation for this test.

type Calculator() = 
  member x.Add expression = 
    match expression with
    | "" -> 0
    | _ -> 1

Everything passed. So now on to the next scenario other subsequent number strings. I add another test and result condition.

  [<TestCase("1", Result = 1)>]
  [<TestCase("2", Result = 2)>]
  member x.Add_single_number_returns_that_number expression =
    calculator.Add expression

It runs, gets a red light fail, I then implement with this minor addition.

type Calculator() = 
  member x.Add expression = 
    match expression with
    | "" -> 0
    | _ -> Int32.Parse expression

Before moving on, I’m just going to cover some of the syntax I’ve been using. The | delimits individual matches, individual discriminated union cases, and enumeration values. In this particular case I’m just using it to match the empty string or the
wildcard. Which speaking of, the _ is a wildcard match or specifies a generic parameter. To learn more about these in detail check out match expressions or generics. There are lots of good things in there.

The other syntax is somewhat more self-explanatory so I’m going to leave it as is for the moment. It is, in the end, when executing the tests evident what is going on at least. Alright, back to the kata. Let’s actually add two numbers. For the test I’m just going to add another TestCase with two actual numbers.

  [<TestCase("1", Result = 1)>]
  [<TestCase("2", Result = 2)>]
  [<TestCase("1 2", Result = 3)>]
  member x.Add_single_number_returns_that_number expression =
    calculator.Add expression

Fails, so on to implementation. I’m just going to do this the cheap “it works” way and do something dumb.

type Calculator() = 
  member x.Add expression = 
    match expression with
    | "" -> 0
    | _ when expression.Contains " " -> 3
    | _ -> Int32.Parse expression

That’ll give me a passing green light, but I’ll add another bit of attribute to the test and get another failing test.

  [<TestCase("1", Result = 1)>]
  [<TestCase("2", Result = 2)>]
  [<TestCase("1 2", Result = 3)>]
  [<TestCase("2 3", Result = 5)>]
  member x.Add_single_number_returns_that_number expression =
    calculator.Add expression

I’ll add the following code to implement and get a passing test.

type Calculator() = 
  member x.Add expression = 
    match expression with
    | "" -> 0
    | _ when expression.Contains " " -> 
        let numbers = expression.Split [| ' ' |]
        (Int32.Parse numbers.[0]) + (Int32.Parse numbers.[1])
    | _ -> Int32.Parse expression

Ok. So that part of the match looks for an empty space, and then takes the two numbers opposite sides of that empty space (array item 0 and 1) and then parses them and adds them together. Keep in mind that ‘ ‘ signifies a single character, and not a string, even though for the contains method that executes on a string, passing in a string with ” ” is ok and the appropriate actions are taken by the compiler.

For the tests I’m going to do a refactor and break them apart just a bit and rename them using the “ xyz “ technique of methods. After the refactor the code looked like this. I got this idea from the “Use F# to write unit tests with readable names” tip.

type CalculatorTests() =
  member x.``should return zero if no string value is passed in.``() =
    let result = calculator.Add ""
    Assert.That(result, Is.EqualTo 0)

  [<TestCase("1", Result = 1)>]
  [<TestCase("2", Result = 2)>]
  member x.``take one number and return that number`` expression =
    calculator.Add expression

  [<TestCase("1 2", Result = 3)>]
  [<TestCase("2 3", Result = 5)>]
  member x.``add single number to single number and return sum`` expression =
    calculator.Add expression

At this point I’m going to take a break, and wrap this up in a subsequent part of this series. It’s been a fun troubleshooting and getting started string calculator kata. So stay tuned and I’ll be slinging some F# math at ya real soon.


_______1 |> F# – Getting Started, Thinking Functionally (Notes)

Recently I took the plunge into writing F# on OS-X and Linux (Ubuntu specifically). This is the first of a new series I’m starting (along with my other ongoing series on JavaScript workflow and practices). In this article particularly I’m just going to provide an overview and notes of key paradigms in functional programming. Most of these notes are derived from a great series called “Thinking Functionally“.

#1 Thinking Functionally: Introduction

This is the article that kicks off the series. The emphasis is focused around realizing that a different way of thinking needs applied when using a functional language. The difference between functional thinking and imperative. The key topics of the series includes:

  • Mathematical Functions
  • Functions and Values
  • Types
  • Functions with Multiple Parameters
  • Defining Functions
  • Function Signatures
  • Organizing Functions

#2 Mathematical Functions

Functional programming, its paradigms and its origins, are all rooted in mathematics. A mathematical function looks something like this:

Add1(x) = x + 1

  • The set of values that can be used as input to the function is called the domain.
  • The set of possible output values from the function is called the range.
  • The function is said to map the domain to the range.

If F# the definition would look like this.

let add1 x = x + 1

The signature of the function would look like this.

val add1 : int -> int

Key Properties of Mathematical Functions

  • A function always gives the same output value for a given input value.
  • A function has no side effects.

Pure functions

  • They are trivially parallelizable.
  • I can use a function lazily.
  • A function only needs to be evaluated once. (i.e. memoization)
  • The function can be evaluated in any order.

Prospectively “Unhelpful” properties of mathematical functions

  • The input and output values are immutable.
  • A function always has exactly one input and one output

#3 Function Values and Simple Values

Looking at this function again:

let add1 x = x + 1

The x means:

  • Accept some value from the input domain.
  • Use the name “x” to represent that value so that we can refer to it later.

It is also referred to as x is bound to the input value. In this binding it will only ever be that input value. This is not an assignment of a variable. Emphasis on the fact that there are no “variables”, only values. This is a critical part of functional thinking.

Function Values: this is a value that provides binding for a function to a name.

Simple Values: This is what one might think of as constants.

let c = 5

Simple Values vs. Function Values

Both are bound to names using the keyword let. The key difference is a function needs to be application to an argument to get a result, and a simple value doesn’t, as it is the value it is.

“Values” vs. “Objects”

In F# most things are referred to as “values”, contrary to “objects” in C# (or other languages like JavaScript for that matter). A value is just a member of a domain, such as a domain of ints, string, or functions that map ints to strings. In theory a value is immutable and has no behavior attached to it. However in F#, even some primitive values have some object-like behavior such as calling a member or property with dot notation syntax like “theValue”.Length or something similar.

Naming Values

Most of the naming is the general ruleset that applies to practically every language. However there is one odd feature that comes in handy in some scenarios. Let’s say you want to have a name such as “this is my really long and flippantly ridiculous name” for a domain. Well, what you can use is the “this is my really long and flippantly ridiculous name“ to have that be the name. It’s a strange feature, but I’ll cover more of it in subsequent articles.

In F# it is also practice to name functions and values with camel case instead of pascal case (camelCase vs. PascalCase).

#4 How Types Work with Functions

Function signatures look like this, with the arrow notation.

val functionName : domain -> range

Example functions:

let intToString x = sprintf "x is %i" x  // format int to string
let stringToInt x = System.Int32.Parse(x)

Example function signatures:

val intToString : int -> string
val stringToInt : string -> int

This means:

  • intToString has a domain of int which it maps onto the range string.
  • stringToInt has a domain of string which it maps onto the range int.

Primitive Types

Standard known types you’d expect: string, int, float, bool, char, byte, etc.

Type Annotations

Sometimes type might not be inferred, if that is the case, the compiler can take annotations to resolve type.

let stringLength (x:string) = x.Length   
let stringLengthAsInt (x:string) :int = x.Length 

Function Types as Parameters

A function that takes other functions as parameters, or returns a function, is called a higher-order function (sometimes abbreviated as HOF).


let evalWith5ThenAdd2 fn = fn 5 + 2 

The signature looks like:

val evalWith5ThenAdd2 : (int -> int) -> int

Looking at an example executing. The example:

let times3 x = x * 3
evalWith5ThenAdd2 times3

The signature looks like:

val times3 : int -> int
val it : int = 17

Also these are very sensitive to types, so beward.

let times3float x = x * 3.0
evalWith5ThenAdd2 times3float

Gives an error:

error FS0001: Type mismatch. Expecting a int -> int but 
              given a float -> float

Function as Output

A function value can also be the output of a function. For example, the following function will generate an “adder” function that adds using the input value.

let adderGenerator numberToAdd = (+) numberToAdd

Using Type Annotations to Constrain Function Types

This example:

let evalWith5ThenAdd2 fn = fn 5 +2

Becomes this:

let evalWith5AsInt (fn:int->int) = fn 5

…or maybe this:

let evalWith5AsFloat (fn:int->float) = fn 5

The “unit” Type

When a function returns no output, it still requires a range, and since there isn’t a void function in mathematical functions this supplants the void as return output. This special range is called a “unit” and has one specific value only, a “()”. It is similar to a void type or a null in C# but also is not, in that it is the value “()”.

Parameterless Functions

For a WAT moment here, parameterless functions come up. In this example we might expect “unit” and “unit”.

let printHello = printf "hello world" 

But what we actually get is:

hello world
val printHello : unit = ()

Like I said, a very WAT kind of moment. To get what we expect, we need to force the definition to have a “unit” argument, as shown.

let printHelloFn () = printf "hello world"

Then we get what we expect.

val printHelloFn : unit -> unit

So why is this? Well, you’ll have to dive in a little deeper and check out the F# for fun and profitThinking Functional” entry on “How Types Work with Functions” for the full low down on that. Suffice it I’ve documented how you get what you would expect, the writer on the site goes into the nitty gritty of what is actually happening here. Possibly, you might have guessed what is happen already. There is also some strange behavior that takes place with forcing unit types with the ignore function that you may want to read on that article too, but if you’re itching to move on, and get going faster than digging through all of this errata, keep going. I’m going to jump ahead to the last section I want to cover for this blog entry of notes, currying.

#5 Currying

Haskell Curry

Haskell Curry

Breaking multi-parameter functions into smaller one-parameter functions. The way to write a function with more than one parameter is to use a process called currying. For more history on this check out Haskell Curry the mathematician to dive deep into his work (which also covers a LOT of other things beside merely currying, like combinatory logic and Curry’s paradox for instance).

Basic example:

let printTwoParameters x y = 
   printfn "x=%i y=%i" x y

This of course looks neat enough right? Well, if we look at the compiler rewrite, it gets rather interesting.

let printTwoParameters x =
   let subFunction y = 
      printfn "x=%i y=%i" x y