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help needed: what am i doing wrong here?
f#

i was happy with this first snippet...

let loop1 n f =

....for a in 1 to n do

........f a

loop1 100 (printfn "Hello World %i")

Which produced:

val unloop : int -> (int -> unit) -> unit

Hello World 1

Hello World 2

Hello World 3

Hello World 4

Hello World 5

....down to "Hello World 100"

But I was confused by the result of this modification:

let loop2 n f =

....for a in 1 to n do

........f

loop2 100 (printfn "Hello World")

Which returned merely:

val loop2 : int -> unit -> unit

Hello World

(Just once)

Rather than the:

Hello World

Hello World

Hello World

Hello World ... so on for 100 lines, that I was expecting.

Can someone please try to explain what I'm missing here?

I know it's not F# misbehaving, but rather my assumptions.

Any help much appreciated!

(dots used to indicate indentation -- #light assumed)

.

The problem is a function without a function isn't a function its just a value. So functions that you don't want to have any parameters have a special parameter called "unit", which is represented by a pair of brackets - ().

So you need to write:

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#light
let loop2 n f = 
    for a in 1 to n do 
        f() 

loop2 100 (fun () -> printfn "Hello World")
By on 1/22/2008 4:58 AM ()

Ah, got it.

Now I can also get to the 3rd step i was trying to reach:

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#light

let loop3 n f = 

....for a in 1 to n do 

........f a

loop3 100 (fun a -> printfn "Hello World %i" a)        

loop3 100 (fun _ -> printfn "Hello World")
By on 1/22/2008 5:10 AM ()

Okay -- moving a little further into my tinkering here, i've got another question...

using loop2 again:

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#light

let loop2 n f = 

....for a in 1..n do 

........f a

i've made a "higher order function" called loop100

let loop100 = loop2 100 //loop2 is partially applied

hence:

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loop100 (fun _ -> printfn "Hello World")

Gives me 100 times "Hello World"

All good so far.

Now I want to create a new higher order function, 

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loop100Who

Such that:

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loop100Who "Fred"

returns:

Hello Fred

Hello Fred

Hello Fred (so on 100 times)

I can't quite craft a loop100Who that does what I want.

I know it will include 'printfn "Hello %s" name'

but my attempts so far do not type check, let alone work as desired... 

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//Some of my dodgy attempts include:

//let loop100Who name = (fun name -> (loop100 (fun _ name -> printfn "Hello %s" name)))

//let loop100Who name = (loop100 (fun _ -> printfn "Hello %s" name) name)

//let loop100Who name = loop100 (fun _ name -> printfn "Hello %s" name) name

//let loop100Who name = loop100 (fun name -> (fun _ -> printfn "Hello %s" name) )

Again -- any help very much appreciated ;-)

By on 1/22/2008 5:56 AM ()

You were very close, its actually simpler that you think. The name is already caught in the lambda so there's no need to try and pass it in again.

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#light
let loop2 n f = 
    for a in 1..n do 
        f a 
        
let loop100 = loop2 100 

let loop100Who name = loop100 (fun _ -> printfn "Hello %s" name)
By on 1/22/2008 7:44 AM ()

Ah! thanks Robert.

By on 1/22/2008 1:56 PM ()

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