F# Immutability, pure function and side effect -

i'm new f# , i'm coding little challenges learn nitty-gritty details language. think have problem because of immutability.

scenario: have read height lines in console, each line contains 1 integer. integer represent size of mountain. after reading input need write line number of highest mountains. if index given highest mountain size set 0 else loose. repeat scenario until mountains have size set zero.

here code wrote:

open system  type mountain = {id:int; height:int}  let readlineint() = int(console.in.readline()) let readmountaindata id = {id = id; height = readlineint()} let readallmountainsdata = [ in 0 .. 7 yield readmountaindata ]  let rec mainloop () =      let mountains = readallmountainsdata      let highestmountain = mountains |> list.maxby (fun x -> x.height)       printfn "%i" highestmountain.id      mainloop()  mainloop() 

this code going infinite loop, believe it's because

let readlineint() = int(console.in.readline()) 

is immutable, value set once , after it's never stop again read line. try put 'mutable' keyword

let mutable readallmountainsdata = [ in 0 .. 7 yield readmountaindata ] 

but didn't change thing. have idea?

edit: know code going infinite loop because after adding logging main loop follow:

let rec mainloop () =      let mountains = readallmountainsdata      console.error.writeline("mountain count:{0} ", mountains.length)      mountains |> list.iter (fun x -> console.error.writeline("mountain id:{0} height:{1}", x.id, x.height))      let highestmountain = mountains |> list.maxby (fun x -> x.height)       printfn "%i" highestmountain.id      mainloop() 

then have in output:

standard error stream:  mountain count:8  mountain id:0 height:9 mountain id:1 height:8 mountain id:2 height:7 mountain id:3 height:6 mountain id:4 height:5 mountain id:5 height:4 mountain id:6 height:3 mountain id:7 height:2 mountain count:8  mountain id:0 height:9 mountain id:1 height:8 mountain id:2 height:7 mountain id:3 height:6 mountain id:4 height:5 mountain id:5 height:4 mountain id:6 height:3 mountain id:7 height:2 mountain count:8  mountain id:0 height:9 mountain id:1 height:8 mountain id:2 height:7 etc... 

why want reread value? because values provided external source. workflow follow:

loop one: read 8 values height of mountains in console output value of highest mountain  loop two: read 8 values height of mountains in console output value of highest mountain  loop three: read 8 values height of mountains in console output value of highest mountain  etc 

let readlineint () = ... defines function. it's body executed every time call it. , in case body has side-effect , side-effect (reading stdin) executed every time body executed. that's not problem.

readallmountainsdata defined list containing data of 7 mountains. each of mountains have own height (because readlineint() called once each mountain). list calculated once , not change after that. not re-calculated every time use readallmountainsdata variable, not function (even though name might suggest otherwise). seems sensible re-reading mountain data every time make no sense.

adding mutable keyword definition allows re-assign variable. is, allows write readallmountainsdata <- somenewvalue later in program change variable's value. since never that, nothing changes.

the reason program loops infinitely mainloop calls again. has no exit condition. fix should decide how want loop / under condition want exit, , implement logic accordingly.

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in edit clarified, want re-read values, need make readallmountainsdata function giving parameter list (let readallmountainsdata () = ...) , call function. way you'll new data on each iteration, loop still infinite unless add exit condition.