Conditional Compilation Directives

Fantomas supports formatting F# code that contains conditional compilation directives (#if, #else, #endif). However, there is an important limitation to be aware of.

How Fantomas handles directives

Fantomas needs to parse your code into an abstract syntax tree (AST) before it can format it. The F# parser processes #if / #else / #endif directives at parse time, meaning it picks one branch based on which defines are active and ignores the other.

To handle this, Fantomas:

1. Parses your code without any defines to discover all conditional directives.
2. Determines every possible combination of defines.
3. Parses and formats the code once for each combination.
4. Merges the results back together.

The limitation: all define combinations must produce valid syntax

Because Fantomas parses your code under every define combination, each combination must result in a valid syntax tree.

For example, the following code cannot be formatted:

module F =
    let a: string =
        #if FOO
        ""
        #endif
        #if BAR
        "a"
        #endif

    let baz: unit = ()

When neither FOO nor BAR is defined, the code becomes:

module F =
    let a: string =

    let baz: unit = ()

This is not valid F# — let a has no body — so the parser raises an error and Fantomas cannot proceed.

How to fix it

Make sure that every combination of defines still produces valid F# code. The most common fix is to add an #else branch:

module F =
    let a: string =
        #if FOO
        ""
        #else
        "a"
        #endif

    let baz: unit = ()

Now, regardless of whether FOO is defined, the parser always sees a complete let binding.

Using .fantomasignore

If you cannot restructure the directives (e.g. because the code is generated or must match a particular pattern), you can exclude the file from formatting using a .fantomasignore file.