Why F# Type Provider?

F# type provider is a very nice feature. I was asked several times why I need type provider? Just because of the intellisense? I believe there should be something else. In the current team, I found people "hate" type, they prefer to use C# object all the times. Now I realize why type is so important and why type can increase code quality.

I want to use a simple approval process as a sample. For my understanding, a type defines the meaning of a area of memory and certain operations which can be executed on this type.

The sample is simple. You can have something to be approved by lead, manager, and director. If a lead approves, it returns a manager type. If manager disapproves, the process will stop and director won't know it. After director's approval, this process stops. The type provider can have three types, director's approve method returns nothing while other type's approve method return higher level type. Lead's approve method returns manager, and manager's approve method returns director.

I'd bet people can say I can use a single type to do the same job. A general class can be used to create three instances. The director instance's approve method can return NULL or any special value to stop the approval process. It can solve this problem provides you know director's approval will stop this process beforehand. A runtime error is not far away if this information is not known beforehand. The type provider uses type to provide the information. After you invoke director's approve method, which returns NULL, user will know immediately that this process is terminated.

I would say in this case, type is a way to move runtime check/error to compile time. The attached is the demo project for this approval process.

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F# on Algorithms - merge sort

Not every Saturday is a pleasant sunshine blue sky. Today is one of these Saturdays.

The real driving power for F# is business requirements from the community and the success of F# depends on the success of community. I was hoping these materials can  inspires discussions around F# and eventually increase the adoption of F#. Our team, like me, wants to serve the community better, so please tell me, your code has bug, your code can be more concise, or your code is shit. 

So please, together, we can make it.

open System

let rec merge list0 list1 = 

    match list0, list1 with 

    | [], [] -> []

    | [], list

    | list, [] -> list

    | h0::t0, h1::t1 -> 

        if h0 < h1 then

            [h0] @ merge t0 list1

        else

            [h1] @ merge list0 t1

let rec mergeSort list = 

    match list with

    | [] -> []

    | [a] -> [a]

    | [a; b] -> [ min a b; max a b ]

    | _ -> 

        let firstHalf = 

            list 

            | > Seq.take (list.Length / 2)

            | > Seq.toList

        let secondHalf = 

            list

            | > Seq.skip (list.Length / 2)

            | > Seq.toList

        let sortedFirstHalf = mergeSort firstHalf

        let sortedSecondHalf = mergeSort secondHalf

        merge sortedFirstHalf sortedSecondHalf

mergeSort [1;3;9;2;4;7;6;5]

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Use Space in F# Namespace and Module

It is Friday's afternoon and sun finally decide to visit Seattle. I decide to have some fun with F#. I tried to use  space in the variable name with ``. It works perfectly and then  I decide to add space to other components like namespace and module.

Result: both namespace and module name can contains space.

But there are some limitation. The C# IDE does not support namespace contains space. As a result, the C# cannot reference the F# function inside a namespace which contains space. Because F# support space, so you can reference function inside F# code.

In File1.fs

namespace A.``My Space``
type MyClass() = class end

In File2.fs, the following code compiles.

module File2

open A.``My Space``
let c = MyClass()
let d = A.``My Space``.MyClass()

Not sure if somebody wants to use this feature to hide the code from C#, but it is do-able.. :-)

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Self Note: Yahoo Finance Tags

Sample to use the following tags:

http://finance.yahoo.com/d/quotes.csv?s=GE+PTR+MSFT&f=snd1l1yr

a	Ask	a2	Average Daily Volume	a5	Ask Size
b	Bid	b2	Ask (Real-time)	b3	Bid (Real-time)
b4	Book Value	b6	Bid Size	c	Change & Percent Change
c1	Change	c3	Commission	c6	Change (Real-time)
c8	After Hours Change (Real-time)	d	Dividend/Share	d1	Last Trade Date
d2	Trade Date	e	Earnings/Share	e1	Error Indication (returned for symbol changed / invalid)
e7	EPS Estimate Current Year	e8	EPS Estimate Next Year	e9	EPS Estimate Next Quarter
f6	Float Shares	g	Day’s Low	h	Day’s High
j	52-week Low	k	52-week High	g1	Holdings Gain Percent
g3	Annualized Gain	g4	Holdings Gain	g5	Holdings Gain Percent (Real-time)
g6	Holdings Gain (Real-time)	i	More Info	i5	Order Book (Real-time)
j1	Market Capitalization	j3	Market Cap (Real-time)	j4	EBITDA
j5	Change From 52-week Low	j6	Percent Change From 52-week Low	k1	Last Trade (Real-time) With Time
k2	Change Percent (Real-time)	k3	Last Trade Size	k4	Change From 52-week High
k5	Percebt Change From 52-week High	l	Last Trade (With Time)	l1	Last Trade (Price Only)
l2	High Limit	l3	Low Limit	m	Day’s Range
m2	Day’s Range (Real-time)	m3	50-day Moving Average	m4	200-day Moving Average
m5	Change From 200-day Moving Average	m6	Percent Change From 200-day Moving Average	m7	Change From 50-day Moving Average
m8	Percent Change From 50-day Moving Average	n	Name	n4	Notes
o	Open	p	Previous Close	p1	Price Paid
p2	Change in Percent	p5	Price/Sales	p6	Price/Book
q	Ex-Dividend Date	r	P/E Ratio	r1	Dividend Pay Date
r2	P/E Ratio (Real-time)	r5	PEG Ratio	r6	Price/EPS Estimate Current Year
r7	Price/EPS Estimate Next Year	s	Symbol	s1	Shares Owned
s7	Short Ratio	t1	Last Trade Time	t6	Trade Links
t7	Ticker Trend	t8	1 yr Target Price	v	Volume
v1	Holdings Value	v7	Holdings Value (Real-time)	w	52-week Range
w1	Day’s Value Change	w4	Day’s Value Change (Real-time)	x	Stock Exchange
y	Dividend Yield