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Looking into an array or consuming a list? - a question about style
f#functional

I'm reading a binary file, and since File.ReadAllBytes returns an array of bytes, I have taken my data from it using offsets like this:

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let getInt16 (bytes : byte[]) offset = (int bytes.[offset] <<< 8) + int bytes.[offset+1]

let getRecord fileName =
    let fileBytes = File.ReadAllBytes fileName

    let recType = getInt16 fileBytes 0
    let recLen  = getInt16 fileBytes 2
     // ...

Thinking more about it, I wonder whether it's better/cleaner functional style to convert the array into a list and consuming it as I go along, like this:

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let getInt16 = function | (h0::h1::tail : byte list) -> (tail, (int h0 <<< 8) + int h1)

let getRecord fileName =


    let fileBytes = File.ReadAllBytes fileName |> Array.to_list

    let (fileBytes, recType) = getInt16 fileBytes


    let (fileBytes, recLen)  = getInt16 fileBytes


    // ...

Opinions, anyone? The file size is > 150 KB, so the extra bloat caused by using a list doesn't really matter.

.

I would stick with the array. What is the list buying you? Nothing (apart from bad perf) that I can see. The array is a local variable in the getRecord function. A little temporary local mutable state never hurt anyone. :)

By on 4/11/2009 1:12 PM ()

I guess I just associate consuming with functional and offsets/pointers with imperative...

An alternative would be to create a BinFile type, which could be consumed.

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type BinFile(fileName : string) =
    let bytes = File.ReadAllBytes fileName
    let mutable offset = 0
    
    member f.getInt16 =
        let ret = (int bytes.[offset] <<< 8) + int bytes.[offset+1]
        offset <- offset + 2
        ret
 
let getRecord fileName =
    let file = BinFile(fileName)

    let recType = file.getInt16
    let recLen  = file.getInt16
    // ...

Anyway, it turned out to be cleaner to generate a list of (recType, recLen, offsetInFileBytes) in this particular case.

By on 4/12/2009 10:16 AM ()

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