gleam-lang/gleam · error · ErrorType

ErrorType::InvalidEndianness

Error message

ErrorType::InvalidEndianness

What it means

ErrorType::InvalidEndianness: endianness options (big/little/native) are only valid on segments whose type is int, float, utf16, utf16_codepoint, utf32, or utf32_codepoint. If a segment carries an endian option with an incompatible type (e.g. bytes, bits, utf8), the compiler rejects it at the endian option's location.

Solutions

1. Remove the endianness option from bytes/bits/utf8 segments
2. Change the segment's type to one that supports endianness (int, float, utf16, utf32, or their codepoints) if byte order matters
3. Apply endianness per numeric field rather than to whole byte blobs

Example fix

// before
<<blob:bytes-big>>
// after
<<blob:bytes>>

// Endianness only applies to: int, float, utf16, utf32, utf16_codepoint, utf32_codepoint
fn endian_valid(type_str: String) -> Bool {
  ["int", "float", "utf16", "utf32", "utf16_codepoint", "utf32_codepoint"]
  |> list.contains(type_str)
}

When it happens

Trigger: Writing <<data:bytes-big>> or <<str:utf8-little>>; the final match in type_options finds categories.endian set while the segment's type is not one of the byte-order-compatible types.

Common situations: Applying endianness habitually to every segment; believing strings/bytes have byte-order semantics; copy-pasting an int segment's option list onto a bytes segment.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• ErrorType::ConflictingEndiannessOptions
• ErrorType::OptionNotSupportedForTarget
• ErrorType::ConflictingSignednessOptions
• ErrorType::ConflictingSizeOptions
• ErrorType::ConflictingUnitOptions

AI-assisted analysis of gleam-lang/gleam@15b07c7830 (2026-09-14). Data as JSON: /api/errors/b895988bd515a1ac. Report an issue: GitHub.

Appendix: source

Thrown at compiler-core/src/bit_array.rs:245

    match categories {
        SegmentOptionCategories {
            type_:
                None
                | Some(
                    Int { .. }
                    | Utf16 { .. }
                    | Utf32 { .. }
                    | Utf16Codepoint { .. }
                    | Utf32Codepoint { .. }
                    | Float { .. },
                ),
            ..
        } => {}

        SegmentOptionCategories {
            endian: Some(endian),
            ..
        } => return err(ErrorType::InvalidEndianness, endian.location()),

        _ => {}
    }

    // signed and unsigned can only be used with int types
    match categories {
        SegmentOptionCategories {
            type_: None | Some(Int { .. }),
            ..
        } => {}

        SegmentOptionCategories {
            type_: Some(opt),
            signed: Some(sign),
            ..
        } => {
            return err(
                ErrorType::SignednessUsedOnNonInt { type_: opt.label() },

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