F# printf compiler magic

Question

In F#, the compiler is clearly doing some magic to make this work:

printfn "%i %i" 6 7 ;; // good
printfn "%i %i" 6 7 8;; // error

How is it doing this? and is there any way to achieve a similar behavior, from within the language?

Ramon Snir · Accepted Answer · 2013-06-14T13:24:41.370

Sadly, this bit of "magic" (as you call it) is hard-coded into the F# compiler. You can extend the compiler, but the result will be non-standard F#.

Here is the specific bit of code that handles that (it isn't too readable, but that's how the F# compiler is written):

and TcConstStringExpr cenv overallTy env m tpenv s  =

    if (AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css m overallTy cenv.g.string_ty) then 
      mkString cenv.g m s,tpenv
    else 
      let aty = NewInferenceType ()
      let bty = NewInferenceType ()
      let cty = NewInferenceType ()
      let dty = NewInferenceType ()
      let ety = NewInferenceType ()
      let ty' = mkPrintfFormatTy cenv.g aty bty cty dty ety
      if (not (isObjTy cenv.g overallTy) && AddCxTypeMustSubsumeTypeUndoIfFailed env.DisplayEnv cenv.css m overallTy ty') then 
        // Parse the format string to work out the phantom types 
        let aty',ety' = (try Formats.ParseFormatString m cenv.g s bty cty dty with Failure s -> error (Error(FSComp.SR.tcUnableToParseFormatString(s),m)))
        UnifyTypes cenv env m aty aty';
        UnifyTypes cenv env m ety ety';
        mkCallNewFormat cenv.g m aty bty cty dty ety (mkString cenv.g m s),tpenv
      else 
        UnifyTypes cenv env m overallTy cenv.g.string_ty;
        mkString cenv.g m s,tpenv

And here is the same code, that also supports numeric strings (i.e. printfn "%i %i" ("4" + 2) "5" will type-check and print 6 5):

and TcConstStringExpr cenv overallTy env m tpenv s  =

    if (AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css m overallTy cenv.g.string_ty) then 
      mkString cenv.g m s,tpenv
    elif (AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css m overallTy cenv.g.int_ty) then 
      mkInt cenv.g m (System.Int32.Parse s),tpenv
    elif (AddCxTypeEqualsTypeUndoIfFailed env.DisplayEnv cenv.css m overallTy cenv.g.int32_ty) then 
      mkInt32 cenv.g m (System.Int32.Parse s),tpenv
    else 
      let aty = NewInferenceType ()
      let bty = NewInferenceType ()
      let cty = NewInferenceType ()
      let dty = NewInferenceType ()
      let ety = NewInferenceType ()
      let ty' = mkPrintfFormatTy cenv.g aty bty cty dty ety
      if (not (isObjTy cenv.g overallTy) && AddCxTypeMustSubsumeTypeUndoIfFailed env.DisplayEnv cenv.css m overallTy ty') then 
        // Parse the format string to work out the phantom types 
        let aty',ety' = (try Formats.ParseFormatString m cenv.g s bty cty dty with Failure s -> error (Error(FSComp.SR.tcUnableToParseFormatString(s),m)))
        UnifyTypes cenv env m aty aty';
        UnifyTypes cenv env m ety ety';
        mkCallNewFormat cenv.g m aty bty cty dty ety (mkString cenv.g m s),tpenv
      else 
        UnifyTypes cenv env m overallTy cenv.g.string_ty;
        mkString cenv.g m s,tpenv

P.S.: I wrote this a long time ago, so I can't remember why there are both mkInt and mkInt32 there. It might be necessary, and might not - but I do remember that this code worked.

@AK_ The original source can be found here: https://github.com/fsharp/fsharp/blob/master/src/fsharp/tc.fs#L6116 — Ramon Snir, Jun 14 '13 at 13:47
@AK_ http://stackoverflow.com/questions/88302/macro-support-in-f . Not really, but if we're already opening the F# compiler: http://ramon.org.il/wp/2011/12/text-based-macro-system-for-f/ — Ramon Snir, Jun 14 '13 at 14:04

score 4 · Answer 2 · answered Jun 14 '13 at 15:19

The magic is in the implicit conversion from a string literal to the type PrintfFormat<_,_,_,_>. For instance, printf takes an argument of type TextWriterFormat<'a>, which is actually just an alias for PrintfFormat<'a,System.IO.TextWriter,unit,unit>.

The magic for this implicit conversion can't be easily emulated within the language, but there's nothing special about the printf family of functions - you can write your own functions that take arguments of type PrintfFormat<_,_,_,_> and use them with string literals without any problem.

While there's no way to extend the implicit conversion from strings, one alternative would be to use type providers. It would be quite easy to write a type provider so that

PrintfTypeProvider<"%i %i">.Apply

returns a value of type int -> int -> string, for instance, but you could also extend the logic in fairly arbitrary ways if you wanted.

score 1 · Answer 3 · answered Aug 31 '13 at 19:21

1

If you just want this behavior on strings without them being output, then sprintf is a similar function that returns the string rather than printing it out. So you can have functions that return strings that are formatted with type-checking this way.

answered Aug 31 '13 at 19:21

Danny Tuppeny

40,147
24
151
275

F# printf compiler magic

3 Answers3