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Getting started with recursion and pattern-matching
f#

Hello

Context:
I'm trying to learn a bit of F# by translating the SML code in Andrew Kennedy's tree-drawing paper into F#, and running into a problem or two!

Here's the simplest problematic example that I have been able to cook up:

let rec size x =
match x with
| [] -> 0
| _ :: tail -> 1 + (size tail);;

size [];;
size [1];;
size [1,2];;

when pasted iunto the F# Interactive window yields

val size : 'a list -> int [Ok]

> val it : int = 0 [Ok]
> val it : int = 1 [Ok]
> val it : int = 1 [Expected: 2]

Notes:
- [Notes in square brackets are mine]
- I'm using F# Version 1.1.12.5 under VS 2005
- Not using the #light option

What I am I missing?

-- Dan

--
Dr Daniel Prager
Senior Software Developer
www.austhink.com

.

The implmentation of size is correct. When you call size [1,2] you are passing it a list containing tuple of two intergers, this counts as one item, thus the result. What you probably wanted to do is "size [1;2];;". [1;2] is a list containing two items, because list items are seperated by ; not , .

Cheers, Rob

By on 11/17/2006 1:30 AM ()

Sorry missed the fact there was alread a reply ... had an early start this morning :(

By on 11/17/2006 1:33 AM ()

Hi Dan,

Lists are enumerated with ";". Tuples are constructed with ",". F# thinks that [1,2] is a list containing 1 tuple and therefore counts the size (as matched) as 1.

If you execute "size [ 1 ; 2 ];;" you should get the desired result of a size of 2.

HTH,

---O

By on 11/16/2006 2:18 PM ()

I see:

[1,2] is treated as an abbreviation for [(1,2)], rather than a synonym for [1;2].

Thanks!

-- Dan

By on 11/16/2006 6:46 PM ()

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