f# - This expression was expected to have type xxxx but here has type unit -

the following code has type error both of printfns.

how should fix it? i'd seq.map loop continue if 1 of items fails.

// files should seq of record  let files = seqofstrs |> seq.map(fun s ->     match s     | helper.parseregex "(\w+) xxxxx" month ->         let currentmonth =  .....         if currentmonth = month.[0]             doc.load(shelf)             // parse doc , return record type. omitted         else             printfn "expect %s found %s." currentmonth month.[0] //error     | _ ->         printfn "no '(month) payment data on line' prompt." //error 

the problem different branches of logic producing different things. in "omitted" section you're apparently returning value of type xxxx (e.g. string), in other branches printfn calls, you're returning without value, in f# represented unit , in other languages called void.

if didn't want loop continue, simplest answer throw exception in cases, e.g:

failwithf "expect %s found %s." currentmonth month.[0] 

the static return type of failwithf can type @ all, including string, because never returns doesn't have produce value of type.

given want code continue, instead return empty string or kind of failure value, e.g.:

printfn "expect %s found %s." currentmonth month.[0] //error "failed" 

now branches have same type code compile, have careful callers don't accidentally interpret value being valid result, why throwing exception cleaner.

a cleaner approach use option value represent success or failure, i.e. return some "..." in correct case , none in error cases. approach nicer when failure common-place occurrence calling code handle rather reporting problem user , aborting:

// files should seq of record  let files = seqofstrs |> seq.map(fun s ->     match s     | helper.parseregex "(\w+) xxxxx" month ->         let currentmonth =  .....         if currentmonth = month.[0]             doc.load(shelf)             // parse doc , produce record type.             record         else             printfn "expect %s found %s." currentmonth month.[0] //error             none     | _ ->         printfn "no '(month) payment data on line' prompt." //error         none 

you need decide want files contain @ point - want explicit none value show failed, or want sequence contain correct values , omit failures entirely?

if want explicit values, can leave option values in place , you'll have seq<string option> result. note type syntactic sugar seq<option<string>>, reflecting f#'s o'caml heritage.

if want omit them, replace seq.map call seq.choose, drop none values , strip off some wrappers, leaving seq<string> current code trying produce.