# Understanding F# map/bind

This blog post aims to explain map/bind in F# in a code only example. It took me a while so I’m writing it up here so I can come back to it and re-read it for the 100th time most likely!

```
let add x y =
x + y
let subtract x y =
x - y
let multiply x y =
x * y
let divide x y =
try
Ok(y / x) //This could blow up with DivideByZero exception
with ex -> Error ex
let homeRolledMap fn result =
match result with
| Ok s -> Ok(fn s)
| Error f -> Error f
let homeRolledBind fn result =
match result with
| Ok s -> fn s
| Error f -> Error f
[<EntryPoint>]
let main argv =
//Understanding Result<int,exn>
add 10 5 |> printfn "10+5 is %i"
subtract 10 5 |> printfn "10-5 is %i"
multiply 10 5 |> printfn "10*5 is %i"
let divResult = divide 5 10
//We have to pattern match the result because it won't be an int it will be Result<int,exn>
match divResult with
| Ok res -> printfn "10/5 is %i" res
| Error err -> printfn "oops"
//Let's chain some method calls together
//add wants two integer inputs but because we called divide which returned Result<int,exn>
//we need to call Result.map to match on the success value of a call to divide and then call add and then return a Result type
//because once we have a Result type in the chain we have to continue with that
let mapRes =
multiply 10 10
|> divide 2
|> Result.map (add 50)
match mapRes with
| Ok res -> printfn "10*10/2+50 is %i" res
| Error err -> printfn "oops"
//An example of why that is beneficial is because if we call divide 0 this will also return a Result<int,exn> but this time it will have errored
//map here will recognise that it doesnt have a success value but an exn/failure value and not call the success function and it will do this numerous times based on the
//numerous functions we call
let mapRes =
multiply 10 10
|> divide 0
|> Result.map (add 50)
|> Result.map (subtract 10)
|> Result.map (add 5)
match mapRes with
| Ok res -> printfn "10*10/0+50-10+5 is %i" res
| Error err -> printfn "oops %A" err
//divide wants two integer inputs but because we called divide previously which returned Result<int,exn>
//we need to call Result.bind to match on the success value of a call to divide and then call divide but then NOT to return a Result type as divide returns us a Result type already
//if we called Result.map instead of bind we'd get back Result<Result<int,exn>,exn>
let mapRes =
multiply 10 10
|> divide 2
|> Result.bind (divide 5)
match mapRes with
| Ok res -> printfn "10*10/2/5 is %i" res
| Error err -> printfn "oops %A" err
//what would it look like if we didnt have map/bind types?
//Once you hit divide you will have to pattern match the result and break up the flow of chained function calls
let multiRes = multiply 10 10 |> divide 2
match multiRes with
| Ok res ->
let newres = res |> add 50
printfn "10*10/2+50 is %i" newres
| Error err -> printfn "oops %A" err
//But what if you want subsequent calls to divide, you'll have to split up the pattern matching which is ugly
let multiRes = multiply 10 10 |> divide 2
match multiRes with
| Ok res ->
let newres =
res
|> add 50
|> divide 2
match newres with
| Ok subsequentres -> printfn "10*10/2+50/2 is %i" subsequentres
| Error err -> printfn "oops %A" err
| Error err -> printfn "oops %A" err
//What if you wanted to call divide 5 times, nested hell!
let firstRes = multiply 10 10 |> divide 2
match firstRes with
| Ok res ->
let secondRes =
res
|> add 50
|> divide 2
match secondRes with
| Ok res ->
let thirdRes =
res
|> subtract 10
|> divide 2
match thirdRes with
| Ok res ->
let fourthRes =
res
|> add 80
|> divide 2
match fourthRes with
| Ok res ->
let fifthRes =
res
|> add 25
|> add 5
|> divide 2
match fifthRes with
| Ok res -> printfn "10*10/2+50/2+80/2+25+5/2 = %i" res
| Error err -> printfn "oops %A" err
| Error err -> printfn "oops %A" err
| Error err -> printfn "oops %A" err
| Error err -> printfn "oops %A" err
| Error err -> printfn "oops %A" err
//To understand how map/bind works and allow us to write more terse code we could roll our own functions to cope with all the pattern matching
//and see what a map/bind looks like
let homeRolled =
multiply 10 10
|> divide 2
|> homeRolledMap (add 50)
|> homeRolledBind (divide 2)
|> homeRolledMap (subtract 10)
|> homeRolledBind (divide 2)
|> homeRolledMap (add 80)
|> homeRolledBind (divide 2)
|> homeRolledMap (add 25)
|> homeRolledMap (add 5)
|> homeRolledBind (divide 2)
//Luckily as we know there is map and bind on the Result module that we can use rather than rolling our own
let res =
multiply 10 10
|> divide 2
|> Result.map (add 50)
|> Result.bind (divide 2)
|> Result.map (subtract 10)
|> Result.bind (divide 2)
|> Result.map (add 80)
|> Result.bind (divide 2)
|> Result.map (add 25)
|> Result.map (add 5)
|> Result.bind (divide 2)
match res with
| Ok res -> printfn "10*10/2+50/2+80/2+25+5/2 = %i" res
| Error err -> printfn "oops %A" err
0 // return an integer exit code
```