CSV reader

module Csv

open System
open System.IO
open System.Reflection
open Microsoft.FSharp.Reflection

type ColumnAttribute(index:int option,name:string option) =     
    inherit Attribute()     
    let mutable index = index
    let mutable name = name
    new () = ColumnAttribute (None, None)
    member x.Index          
        with get() = match index with | Some i -> i | None -> -1
        and set value = index <- Some value         
    member x.Name          
        with get() = match name with | Some n -> n | None -> ""
        and set value = name <- Some value     

type CsvReader<'a>(typeConverter:Type -> (string -> obj)) = 
    let mutable header = Map.empty
    let recordType = typeof<'a>  
    let fields = FSharpType.GetRecordFields(recordType)
    let objectBuilder = FSharpValue.PreComputeRecordConstructor(recordType)
    let split (delim:char) (line:string) = 
       line.Split([|delim|]) |> Array.map( fun s -> s.Trim())

    member x.CreateRecord(header:Map<string,int>, delim, line) = 
        let lookupFromHeader (column:ColumnAttribute) = 
            match column.Name with
            | name when name <> String.Empty ->
                try
                    Some header.[name]
                with e -> failwithf "no"  
            | _ -> None
             
        let schema = fields |> Array.mapi( fun fieldIndex field -> 
            let propertyInfo = recordType.GetProperty(field.Name)
            let deserializeColumnData = typeConverter field.PropertyType
            let columnIndex = 
                match propertyInfo.GetCustomAttributes(typeof<ColumnAttribute>,false) with
                | [| (:? ColumnAttribute as col) |] -> 
                    match col.Index with
                    | i when i >= 0 -> i 
                    | _ -> 
                        match lookupFromHeader col with
                        | Some(i) -> i
                        | None -> fieldIndex 
                | _ -> fieldIndex
                
            (fieldIndex, field.Name, columnIndex, deserializeColumnData) )
        
        let fieldContentFromSchema (words:string[]) = 
            let deserializedData = 
                schema 
                |> Array.map( fun (fieldIndex, fieldName, columnIndex, deserializeColumnData) -> 
                        deserializeColumnData words.[columnIndex])
            deserializedData

        let words = line |> split delim |> fieldContentFromSchema
        let convertColumn colText (fieldName, deserializeColumnData) =
            try deserializeColumnData colText
            with e ->
                failwithf "error converting '%s' to field '%s'" colText fieldName

        let obj = objectBuilder(words)
        unbox<'a>(obj)
    
    member x.ReadFile(file, separator:char, firstLineHasHeader:bool) = 
        seq { 
            use textReader = File.OpenText(file)
            if firstLineHasHeader then
                header <-
                    textReader.ReadLine() 
                    |> split separator
                    |> Array.filter (fun name -> not (String.IsNullOrWhiteSpace name))
                    |> Array.mapi (fun i name -> (name, i))
                    |> Map.ofArray
            while not textReader.EndOfStream do
                let line = textReader.ReadLine()
                if not (String.IsNullOrEmpty line) then
                    yield x.CreateRecord(header, separator, line)
        }


//Examples:
//the csv-header is mandatory for this case!
type Substance = {
    [<Column(Name="subst id")>] Id : int 
    [<Column(Name="name")>] Name : string
    [<Column(Name="sequence")>] Sequence : string
}

// a one-to-one mapping to the column names
// UPDATE: Dont' use this any more. As of the writing of this snippet I wasn't aware of the fact, that field order isn't guaranted by the reflection mechanism.
type Probe = {
    Name : string
    Mismatches : int
    Feature : string
    HitLocation : string
    Strain : string
} 

//0 based index mapping 
type ProbeAlt = {
    [<Column(Index=4)>]Strain : string
    [<Column(Index=0)>]Name : string
}

//read the csv
let typeConverter _type =
    match _type with
    | t when t = typeof<float>    -> (System.Double.Parse >> box)
    | t when t = typeof<int>      -> (System.Int32.Parse >> box)
    | t when t = typeof<string>   -> (fun(s:string) -> box s)
    | t when t = typeof<bool>     -> (System.Boolean.Parse >> box)
    | t -> failwithf "Unknown type %A" t

let path = "" //....
let reader = new CsvReader<Probe>(typeConverter)
let hasHeader = true
let separator = '\t'
let probes = reader.ReadFile(path, separator, hasHeader)