How can I define this lazy (infinite?) data structure in F#

f#

Solution

You'll have to use an intermediate type:

type GetNext = GetNext of (unit -> char * GetNext)

let rec nextAt index text = 
    if index < String.length text then
        (text.[index], GetNext(fun () -> nextAt (index + 1) text))
    else
        failwith "End of string."

The answers to this question about y combinator explore this limitation in greater depth and pose workarounds.

Problem

I am having a problem defining the following simple text cursor which is represented by a tuple where the first element is a current character and the second one if a function that gets the next one or crashes. ``` let rec nextAt index text = if index < String.length text then (text.[index], (fun() -> nextAt (index + 1) text)) else failwith "End of string." ``` I am getting ``` Error 1 Type mismatch. Expecting a char * (unit -> 'a) but given a 'a The resulting type would be infinite when unifying ''a' and 'char * (unit -> 'a)' ```

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