Are Preprocessor Directives Indentation Sensitive in F#?

Question

FSI version: 11.0.50727.1

So I was working on an F# shell script and I ran across something that sort of surprised me.

When I did this:

#if INTERACTIVE
    #r "System.Data.dll"
    #r "FSharp.Data.TypeProviders.dll"
    #r "System.Data.Linq.dll"
#endif

open System

I got an FS0010 error when I pasted the block into the FSI. But if I did not indent the #r lines, no FS0010 error. I'm just sort of surprised that preprocessor lines would be indentation sensitive. Is this a compiler issue or is there something else at work here?

Answer 1

I think the specification and documentation are quite unclear on this topic, but the specification makes a notable distinction between lexical preprocessor directives and compiler directives (see §12.4):

Compiler directives are declarations in non-nested modules or namespace declaration groups in the following form:

# id string ... string

The lexical preprocessor directives #if, #else, #endif and #indent "off" are similar to compiler directives. For details on #if, #else, #endif, see §3.3. The #indent "off" directive is described in §18.4.

My interpretation is that lexical preprocessor directives are actually hanled by some pre-processor before running the main compilation and so the indentation does not matter for these.

On the other hand, directives like #r, #load, #time etc. are processed later by the compiler and so they need to match the usual F# indentation guidelines.

As @unwind says, the documentation states "Indentation is not significant for preprocessor directives", but I think this applies only to the preprocessor directives listed on that documentation page (which are lexical preprocessor directives and not compiler directives).

Answer 2

According to the documentation, it must be something else at work:

Indentation is not significant for preprocessor directives.