conditional-composition

name	conditional-composition
version	0.22.0
description	Use when a Markout report must adapt to its data — show or hide whole sections, drop table columns that are empty/uniform, filter output to a chosen set of sections, or render one model into several shapes ("one model, many views"). This is Markout's highest-value idiom: declare the conditions with attributes instead of hand-writing if/else + StringBuilder. Requires the base `markout` pattern (model + context + Serialize). Don't decompile the assembly or web-search the API — the conditional idioms are here.

Conditional composition — one model, many views

The trap this skill prevents: hand-rolling if (hasErrors) sb.AppendLine("## Errors") and manual column arithmetic. In Markout you declare the condition on the model; the generator renders the right shape. This keeps one source of truth and makes the same model serve quiet/detail/export modes.

Conditional sections

[MarkoutSerializable(TitleProperty = nameof(Name))]
public class Inspection
{
    public string Name { get; set; } = "";

    // Section renders ONLY when HasFailures is true. Compute the predicate on the model.
    [MarkoutSection(Name = "Failures", ShowWhenProperty = nameof(HasFailures))]
    public List<FailureRow>? Failures { get; set; }
    public bool HasFailures => Failures is { Count: > 0 };

    // EmptyText shows a fallback paragraph when the list is non-null but empty; null omits the section.
    [MarkoutSection(Name = "Warnings", EmptyText = "No warnings.")]
    public List<WarnRow>? Warnings { get; set; }
}

• ShowWhenProperty = nameof(Bool) gates a section on a bool property.
• [MarkoutShowWhen(nameof(Bool))] gates a scalar field the same way.
• [MarkoutSkipNull] / [MarkoutSkipDefault] drop individual fields when null/default.

Adaptive columns — hide what carries no information

// Drop the "Pattern" column when it's uniform/empty across the rows (keeps tables compact).
// The attribute goes on the SECTION list and names a static bool predicate + the column to hide.
[MarkoutSection(Name = "Matches")]
[MarkoutIgnoreColumnWhen(nameof(PatternIsUniform), "Pattern")]
public List<MatchRow>? Matches { get; set; }

public static bool PatternIsUniform(List<MatchRow>? rows)
    => rows?.Select(r => r.Pattern).Distinct().Count() <= 1;

// IgnoreProperty hides named columns unconditionally.
[MarkoutSection(Name = "Debug", IgnoreProperty = "InternalId,Debug")]
public List<MatchRow>? DebugRows { get; set; }

[MarkoutIgnoreColumnWhen(...)] is the declarative replacement for "compute distinct values, then rebuild headers and rows." Columns hidden this way are also dropped from TSV/JSONL decomposition.

Filter to specific sections at render time

var options = new MarkoutWriterOptions { IncludeSections = new HashSet<string> { "Failures" } };
MarkoutSerializer.Serialize(report, Console.Out, new MarkdownFormatter(), ctx, options);

IncludeSections renders only the named sections — the caller-side lever for "just show me X" without a second model, and the declarative way to drive quiet/detail/verbosity views from one model.

One model, many shapes: same-name section variants

When a mode needs a different projection of the same logical section (e.g. terse vs with-docs), declare multiple properties with the same Name and gate them so exactly one renders:

[MarkoutSection(Name = "Members", ShowWhenProperty = nameof(Terse))]
public List<MemberRow>? MembersTerse { get; set; }

[MarkoutSection(Name = "Members", ShowWhenProperty = nameof(WithDocs))]
public List<MemberDocRow>? MembersWithDocs { get; set; }

This "same-name polymorphic section" is how a single report serves --docs, quiet, and select modes without branching in the writer. Set the gating bools when you build the model.

Guardrails

• Prefer declaration over imperative assembly: no if+AppendLine, no manual header/row rebuilding.
• Keep predicates (Has*, Terse) as computed properties on the model, next to the data.
• Empty vs absent matters: null list omits a section; empty list + EmptyText shows the fallback.