Warnings

Fable reports a warning when it compiles code whose behaviour differs from .NET.

Each Fable warning has a code made of FABLE followed by four digits. The code is printed in the compiler output:

src/App.fs(3,0): (3,57) warning FABLE FABLE0100: CultureInfo argument is ignored

Treat warnings as errors#

Set TreatWarningsAsErrors in your project file to fail the compilation on any warning, from F# or from Fable.

<PropertyGroup>
    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
</PropertyGroup>

Suppress a warning#

Write a directive in a comment to suppress a Fable warning. A suppressed warning is not reported and does not fail the build when TreatWarningsAsErrors is enabled.

Directives apply to Fable warnings only. Use #nowarn for F# compiler warnings. Errors are never suppressed.

fable-disable-line#

Suppresses the listed warnings reported on the same line.

"abc".StartsWith("a", true, CultureInfo.InvariantCulture) // fable-disable-line FABLE0100

When the expression spans several lines, the directive can sit on any of them.

"abc".StartsWith(
    "a", true, CultureInfo.InvariantCulture) // fable-disable-line FABLE0100

fable-disable-next-line#

Suppresses the listed warnings reported on the following line.

// fable-disable-next-line FABLE0100
"abc".StartsWith("a", true, CultureInfo.InvariantCulture)

fable-disable and fable-enable#

fable-disable suppresses the listed warnings from its line to the matching fable-enable, or to the end of the file.

// fable-disable FABLE0100
"abc".StartsWith("a", true, CultureInfo.InvariantCulture)
"abc".EndsWith("c", true, CultureInfo.InvariantCulture)
// fable-enable FABLE0100

fable-disable must list the codes it suppresses. A fable-disable without codes suppresses every Fable warning until the end of the file and is reported as FABLE0003.

Syntax#

• Codes are separated by spaces or commas and are case-insensitive: FABLE0100, fable0101.
• A colon after the directive name is accepted: // fable-disable-line: FABLE0100.
• Text after -- is a justification and is ignored: // fable-disable-line FABLE0100 -- culture is irrelevant here.
• Line comments and block comments are both accepted: (* fable-disable-line FABLE0100 *).
• fable-disable-line and fable-disable-next-line without codes suppress every Fable warning on their target line.
• Directives inside #if blocks are read with the same compiler directives as the code, so a directive inside #if FABLE_COMPILER is honoured.

Warnings from inline functions#

A warning raised by an inline function is reported at the definition of the function, not at the call site. Put the directive next to the definition.

let inline startsWithCulture (s: string) =
    s.StartsWith("a", true, CultureInfo.InvariantCulture) // fable-disable-line FABLE0100

startsWithCulture "abc"

Directive warnings#

Fable reports a problem with a directive itself under codes FABLE0001 to FABLE0099. These warnings cannot be suppressed.

Code	Message
FABLE0001	Unknown warning code in a fable-disable directive
FABLE0002	This fable-disable directive doesn't suppress anything
FABLE0003	A fable-disable block must list the warning codes it suppresses

Warning codes#

Codes FABLE0100 and up describe the compiled code and can be suppressed.

The complete list is in WarningCodes.fs in the Fable repository.

Code	Message
FABLE0100	CultureInfo argument is ignored
FABLE0101	String comparison argument is ignored
FABLE0102	Format provider argument is ignored
FABLE0103	NumberStyles argument is ignored
FABLE0104	DateTimeStyles argument is ignored
FABLE0105	TimeSpan precision is limited to milliseconds, microsecond arguments are ignored
FABLE0106	Private representation flag is ignored
FABLE0107	Base64 offset, length and formatting arguments are ignored
FABLE0108	Write will behave as WriteLine
FABLE0109	Async.Start will behave as StartImmediate
FABLE0110	Task.Result and GetAwaiter().GetResult() return the task itself in JS/TS
FABLE0111	Types can only be resolved at compile time. At runtime this will be same as typeof<obj>
FABLE0112	Cannot type test (evals to false)
FABLE0113	Ignoring base call
FABLE0114	Only inheriting from primary constructors is supported
FABLE0115	Inlined argument is being captured but is also used somewhere else. There's a risk of double evaluation
FABLE0116	CaseTest attribute is ignored: it is only supported on TypeScriptTaggedUnion cases
FABLE0117	String.Compare and String.CompareTo will be compiled as String.CompareOrdinal (Rust)
FABLE0118	Rust chars are Unicode scalar values, so surrogate tests will be false
FABLE0119	Support for F# static and module do bindings is disabled by default (Rust)
FABLE0120	Regex flag is not supported in Php