Compile-time constraints for strings in F#, similar to Units of Measure - is it possible?

Question

I'm developing a Web application using F#. Thinking of protecting user input strings from SQL, XSS, and other vulnerabilities.

In two words, I need some compile-time constraints that would allow me discriminate plain strings from those representing SQL, URL, XSS, XHTML, etc.

Many languages have it, e.g. Ruby’s native string-interpolation feature #{...}.
With F#, it seems that Units of Measure do very well, but they are only available for numeric types.
There are several solutions employing runtime UoM (link), however I think it's an overhead for my goal.

I've looked into FSharpPowerPack, and it seems quite possible to come up with something similar for strings:

[<MeasureAnnotatedAbbreviation>] type string<[<Measure>] 'u> = string
// Similarly to Core.LanguagePrimitives.IntrinsicFunctions.retype
[<NoDynamicInvocation>]
let inline retype (x:'T) : 'U = (# "" x : 'U #)
let StringWithMeasure (s: string) : string<'u> = retype s

[<Measure>] type plain
let fromPlain (s: string<plain>) : string =
    // of course, this one should be implemented properly
    // by invalidating special characters and then assigning a proper UoM
    retype s

// Supposedly populated from user input
let userName:string<plain> = StringWithMeasure "John'); DROP TABLE Users; --"
// the following line does not compile
let sql1 = sprintf "SELECT * FROM Users WHERE name='%s';" userName
// the following line compiles fine
let sql2 = sprintf "SELECT * FROM Users WHERE name='%s';" (fromPlain userName)

Note: It's just a sample; don't suggest using SqlParameter. :-)

My questions are: Is there a decent library that does it? Is there any possibility to add syntax sugar?
Thanks.

Update 1: I need compile-time constraints, thanks Daniel.

Update 2: I'm trying to avoid any runtime overhead (tuples, structures, discriminated unions, etc).

Answer 1

A bit late (I'm sure there's a time format where there is only one bit different between February 23rd and November 30th), I believe these one-liners are compatible for your goal:

type string<[<Measure>] 'm> = string * int<'m>

type string<[<Measure>] 'm> = { Value : string }

type string<[<Measure>] 'm>(Value : string) = struct end

Answer 2

In theory it's possible to use 'units' to provide various kinds of compile-time checks on strings (is this string 'tainted' user input, or sanitized? is this filename relative or absolute? ...)

In practice, I've personally not found it to be too practical, as there are so many existing APIs that just use 'string' that you have to exercise a ton of care and manual conversions plumbing data from here to there.

I do think that 'strings' are a huge source of errors, and that type systems that deal with taintedness/canonicalization/etc on strings will be one of the next leaps in static typing for reducing errors, but I think that's like a 15-year horizon. I'd be interested in people trying an approach with F# UoM to see if they get any benefit, though!

Answer 3

The simplest solution to not being able to do

"hello"<unsafe_user_input>

would be to write a type which had some numeric type to wrap the string like

type mystring<'t>(s:string) =
    let dummyint = 1<'t>

Then you have a compile time check on your strings

Answer 4

It's hard to tell what you're trying to do. You said you "need some runtime constraints" but you're hoping to solve this with units of measure, which are strictly compile-time. I think the easy solution is to create SafeXXXString classes (where XXX is Sql, Xml, etc.) that validate their input.

type SafeSqlString(sql) =
  do
    //check `sql` for injection, etc.
    //raise exception if validation fails
  member __.Sql = sql

It gives you run-time, not compile-time, safety. But it's simple, self-documenting, and doesn't require reading the F# compiler source to make it work.

But, to answer your question, I don't see any way to do this with units of measure. As far as syntactic sugar goes, you might be able to encapsulate it in a monad, but I think it will make it more clunky, not less.

Answer 5

You can use discriminated unions:

type ValidatedString = ValidatedString of string
type SmellyString = SmellyString of string

let validate (SmellyString s) =
  if (* ... *) then Some(ValidatedString s) else None

You get a compile-time check, and adding two validated strings won't generate a validated string (which units of measure would allow).

If the added overhead of the reference types is too big, you can use structs instead.