Design decisions

Fantomas rewrites the entire source text after formatting. Think of it like a word document: Fantomas will re-type your entire text according to its rules in a new file. It does not modify the original text. This approach ensures complete consistency and adherence to the formatting rules, but it means that all formatting decisions are made by Fantomas according to its opinionated style guide.

Because the output has to mean what the input meant, Fantomas sometimes behaves in a way that can look surprising at first: keeping code multiline that would fit on one line, refusing to format a file at all, or laying out a chain of calls in a way you would not have typed by hand. None of this is a bug or a missed opportunity. Each case below explains the reasoning behind one of those behaviors.

Open-ended expressions

Sometimes Fantomas leaves a short record, list or tuple spread over several lines when it would comfortably fit on one, because collapsing it would silently change what the code means. See Open-ended expressions.

Formatting chain expressions

A chain of dotted member access and calls, such as document.Body.FirstChild.AppendChild(newNode), is one of the few shapes where Fantomas has a real choice to make about where line breaks go. See Formatting chain expressions.

Conditional compilation directives

Fantomas formats code under every combination of #if / #else / #endif defines and merges the results, which means every combination has to produce valid F# on its own. See Conditional Compilation Directives.