Enforce can_contain in tree mutation methods

Fix #9.

src/markdown.jl

@@ -484,3 +484,8 @@ Base.:(==)(x::TableCell, y::TableCell) = (x.align == y.align) && (x.header == y.
 # struct Backslash <: AbstractInline end
 # struct SoftBreak <: AbstractInline end
 # struct LineBreak <: AbstractInline end
+
+struct InvalidChildException <: Exception
+    parent :: AbstractElement
+    child :: AbstractElement
+end

src/node.jl

@@ -272,6 +272,7 @@ children.
 """
 function Base.push!(children::NodeChildren{T}, child::T) where {T <: Node}
     node = children.parent
+    assert_can_contain(node, child)
     # append_child
     # The child node is unlinked first
     unlink!(child)
@@ -300,6 +301,7 @@ children.
 """
 function Base.pushfirst!(children::NodeChildren{T}, child::T) where T
     node = children.parent
+    assert_can_contain(node, child)
     # prepend_child
     # The child node is unlinked first
     unlink!(child)
@@ -329,6 +331,7 @@ node. If `sibling` is part of another tree, then it is unlinked from that tree f
 function insert_after!(node::Node, sibling::Node)
     # Adds the sibling after this node:
     isrootnode(node) && throw(ArgumentError("the reference node must not be a root node"))
+    assert_can_contain(node.parent, sibling)
     # The sibling node is unlinked first:
     unlink!(sibling)
     # If there is a node after `node`, we point sibling to it:
@@ -359,6 +362,7 @@ root node. If `sibling` is part of another tree, then it is unlinked from that t
 function insert_before!(node::T, sibling::T) where {T <: Node}
     # Fallback method for insert_before!
     isrootnode(node) && throw(ArgumentError("the reference node must not be a root node"))
+    assert_can_contain(node.parent, sibling)
     # If this node is the first node, then we can prepend the sibling as a child
     # to the parent node. Otherwise, we just insert it after the previous node.
     if isnothing(node.previous)
@@ -397,3 +401,8 @@ function Base.:(==)(x::Node{T}, y::Node{T}) where T
     end
     return true
 end
+
+function assert_can_contain(parent::T, child::T) where {T <: Node}
+    can_contain(parent.element, child.element) && return nothing
+    throw(InvalidChildException(parent.element, child.element))
+end

test/invalidast.jl

@@ -0,0 +1,40 @@
+using MarkdownAST: MarkdownAST, @ast, Node,
+    Document, Paragraph, Strong, Link, CodeBlock, HTMLBlock, Code,
+    InvalidChildException, insert_after!, insert_before!
+using Test
+
+@testset "Invalid AST" begin
+    # Document can only contain blocks, not inlines
+    @test_throws InvalidChildException (@ast Document() do
+        Link("", "")
+    end)
+    valid_document = @ast Document() do
+        Paragraph()
+    end
+    node_strong = Node(Strong())
+    @test_throws InvalidChildException push!(valid_document.children, node_strong)
+    @test_throws InvalidChildException pushfirst!(valid_document.children, node_strong)
+    @test_throws InvalidChildException insert_after!(first(valid_document.children), node_strong)
+    @test_throws InvalidChildException insert_before!(first(valid_document.children), node_strong)
+
+    # Paragraph can only contain inlines, but not blocks:
+    @test_throws InvalidChildException (@ast Paragraph() do
+        HTMLBlock("")
+    end)
+    valid_p = @ast Paragraph() do
+        "..."
+    end
+    node_codeblock = Node(CodeBlock("", ""))
+    @test_throws InvalidChildException push!(valid_p.children, node_codeblock)
+    @test_throws InvalidChildException pushfirst!(valid_p.children, node_codeblock)
+    @test_throws InvalidChildException insert_after!(first(valid_p.children), node_codeblock)
+    @test_throws InvalidChildException insert_before!(first(valid_p.children), node_codeblock)
+
+    # Some nodes can not have child nodes
+    @test_throws InvalidChildException (@ast CodeBlock("", "") do
+        "..."
+    end)
+    @test_throws InvalidChildException (@ast Code("...") do
+        "..."
+    end)
+end

test/runtests.jl

@@ -4,4 +4,5 @@ using Test
 @testset "MarkdownAST" begin
     include("markdown.jl")
     include("node.jl")
+    include("invalidast.jl")
 end