visual studio - Why doesn't F# interactive flag this error? -

i have code few hundred lines. many small pieces of have following structure:

let soa =     election     |> series.observations printfn "%a" <| soa 

frequently 2 things happen:

1) mysteriously last line changed to:

printfn "%a" <| 

so code above , follows becomes

let soa =     election     |> series.observations printfn "%a" <|  let sls =     election     |> series.sample (seq ["party a"; "party r"]) printfn "%a" <| sls 

this happens hundreds of lines above editing file in editor.

2) when happens f# interactive not flag error. no error messages generated. however, if try access sls message:

error fs0039: value or constructor 'sls' not defined.

any ideas on why bit of code erased in editor? (this happens quite frequently)

and why doesn't f# interactive issue error message?

the second let block interpreted argument preceding printfn, because pipe, being operator, provides exception offset rule: second argument of operator not have indented farther first argument. , since second let block not @ top level, rather part of printfn's argument, definitions don't become accessible outside.

let's try experimentation:

let f x = x+1  // normal application f 5    // complex expression argument f (5+6)  // let-expression argument f (let x = 5 in x + 6)  // replacing `in` newline f ( let x = 5     x + 6 )  // replacing parentheses pipe f <|    let x = 5   x + 6  // operators (of pipe one) have exception offset rule. // done support flows this: [1;2;3] |> list.map ((+) 1) |> list.toarray  // applying exception `f` + `let` expression: f <| let x = 5 x + 6