Compatibility with Fable

Fickl compiles Fable projects. You can switch to it for the compiler speed alone, and try its features one at a time.

Fable code keeps working

Fable's attributes and functions stay in the Fable namespaces and keep their meaning. Fickl puts what it adds in its own namespaces, Fickl.Core and Fickl.Core.JS, so nothing changes until you open them.

When Fickl gives an attribute that Fable also has extra options, the options are on the attribute of the same name in Fickl.Core. [<Fable.Core.AttachMembers>] behaves as in Fable; [<Fickl.Core.AttachMembers(false)>] adds the switch to turn it off.

Changes that would alter how existing code behaves at run time stay behind a compiler flag. --strictOptions is the example: it keeps Some null apart from None, as .NET does, which is more correct and also changes what existing code sees. See Compiler flags.

FICKL_COMPILER

Fickl defines FICKL_COMPILER, alongside FABLE_COMPILER. Use it to choose between Fickl.Core and Fable.Core where both have something:

let compiler =
#if FICKL_COMPILER
    "fickl"
#else
    "fable"
#endif

export const compiler = "fickl";

export const compiler = "fickl";

Only the Fickl branch is compiled. Fable skips it and compiles the other.

Behaviour that differs

Some of Fickl's changes are fixes rather than features: they make output match .NET where Fable's does not. Each one is a test case in the repository's tests/Differences, compiled with both compilers and run with Node.js. A case is removed when Fable passes it too. Differences from Fable lists the cases that still differ.

A few changes alter the runtime representation of a type. They are listed in the release notes, with what they break:

• In 0.9, decimal is represented with a native BigInt instead of big.js. JavaScript that calls big.js methods on an F# decimal, such as d.toFixed(2), no longer works.