Attribute Constraints

This adds some attribute constraints. We validate certain constraints
based on information from http://www.graphviz.org/doc/info/attrs.html.
Validation errors are collected as part of builders, we still add the
attribute regardless or not if it fails validation, and build methods
return ValidationResult<T> where errors is a Vec<ValidationError>. We
still add attributes even if the validation fails so that users can
ignore validation errors and build appropriate struct.

4 files changed, 198 insertions, 36 deletions

diff --git a/src/attributes/mod.rs b/src/attributes/mod.rs
index dafbf4a..7f4fb0d 100644
--- a/src/attributes/mod.rs
+++ b/src/attributes/mod.rs
@@ -44,9 +44,11 @@ pub use crate::attributes::style::{EdgeStyle, GraphStyle, NodeStyle, Styles};
 #[doc(hidden)]
 pub use crate::attributes::AttributeText::{AttrStr, EscStr, HtmlStr, QuotedStr};
 use crate::dot::DotString;
+use crate::validation::{ValidationError, ValidationResult};
 use indexmap::map::IndexMap;
 use std::borrow::Cow;
 use std::collections::HashMap;
+use Cow::Borrowed;
 
 /// The text for a graphviz label on a node or edge.
 #[derive(Clone, PartialEq, Eq, Debug)]
@@ -459,10 +461,12 @@ pub trait GraphAttributes<'a> {
         self.add_attribute("fontpath", AttributeText::quoted(font_path))
     }
 
-    // TODO: constrain
     /// Font size, in points, used for text.
     /// default: 14.0, minimum: 1.0
     fn font_size(&mut self, font_size: f32) -> &mut Self {
+        if font_size < 1.0 {
+            self.add_validation_error("fontsize", "Must be greater than or equal to 1.0")
+        }
         Attributes::font_size(self.get_attributes_mut(), font_size);
         self
     }
@@ -605,10 +609,12 @@ pub trait GraphAttributes<'a> {
         self.add_attribute("newrank", AttributeText::from(newrank))
     }
 
-    // TODO: add constraint
     /// specifies the minimum space between two adjacent nodes in the same rank, in inches.
     /// default: 0.25, minimum: 0.02
     fn nodesep(&mut self, nodesep: f32) -> &mut Self {
+        if nodesep < 0.02 {
+            self.add_validation_error("nodesep", "Must be greater than or equal to 0.02")
+        }
         self.add_attribute("nodesep", AttributeText::from(nodesep))
     }
 
@@ -653,6 +659,9 @@ pub trait GraphAttributes<'a> {
     /// Used only if rotate is not defined.
     /// Default: 0.0 and minimum: 360.0
     fn orientation(&mut self, orientation: f32) -> &mut Self {
+        if orientation < 0.0 || orientation > 360.0 {
+            self.add_validation_error("orientation", "Must be between 0 and 360")
+        }
         Attributes::orientation(self.get_attributes_mut(), orientation);
         self
     }
@@ -671,7 +680,6 @@ pub trait GraphAttributes<'a> {
         self.add_attribute("pack", AttributeText::from(pack))
     }
 
-    // TODO: constrain to non-negative integer.
     /// Whether each connected component of the graph should be laid out separately, and then
     /// the graphs packed together.
     /// This is used as the size, in points,of a margin around each part; otherwise, a default
@@ -721,10 +729,12 @@ pub trait GraphAttributes<'a> {
         self.add_attribute("pagedir", AttributeText::from(page_dir))
     }
 
-    // TODO: constrain
     /// If quantum > 0.0, node label dimensions will be rounded to integral multiples of the quantum.
     /// default: 0.0, minimum: 0.0
     fn quantum(&mut self, quantum: f32) -> &mut Self {
+        if quantum < 0.0 {
+            self.add_validation_error("quantum", "Must be greater than or equal to 0")
+        }
         self.add_attribute("quantum", AttributeText::from(quantum))
     }
 
@@ -762,7 +772,6 @@ pub trait GraphAttributes<'a> {
         self.add_attribute("rotate", AttributeText::from(rotate))
     }
 
-    // TODO: constrain
     /// Print guide boxes in PostScript at the beginning of routesplines if showboxes=1, or at
     /// the end if showboxes=2.
     /// (Debugging, TB mode only!)
@@ -874,6 +883,8 @@ pub trait GraphAttributes<'a> {
     ) -> &mut Self;
 
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>>;
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str);
 }
 
 impl<'a> GraphAttributes<'a> for GraphAttributeStatementBuilder<'a> {
@@ -898,21 +909,37 @@ impl<'a> GraphAttributes<'a> for GraphAttributeStatementBuilder<'a> {
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>> {
         &mut self.attributes
     }
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str) {
+        self.errors.push(ValidationError {
+            field: Borrowed(field),
+            message: Borrowed(message),
+        })
+    }
 }
 
 // I'm not a huge fan of needing this builder but having a hard time getting around &mut without it
 pub struct GraphAttributeStatementBuilder<'a> {
     pub attributes: IndexMap<String, AttributeText<'a>>,
+    errors: Vec<ValidationError>,
 }
 
 impl<'a> GraphAttributeStatementBuilder<'a> {
     pub fn new() -> Self {
         Self {
             attributes: IndexMap::new(),
+            errors: Vec::new(),
         }
     }
 
-    pub fn build(&self) -> IndexMap<String, AttributeText<'a>> {
+    pub fn build(&self) -> ValidationResult<IndexMap<String, AttributeText<'a>>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
+        }
+        Ok(self.build_ignore_validation())
+    }
+
+    pub fn build_ignore_validation(&self) -> IndexMap<String, AttributeText<'a>> {
         self.attributes.clone()
     }
 }
@@ -1112,10 +1139,12 @@ impl Attributes {
 }
 
 pub trait NodeAttributes<'a> {
-    // TODO: constrain
     /// Indicates the preferred area for a node or empty cluster when laid out by patchwork.
-    /// default: 1.0, minimum: >0
+    /// default: 1.0, minimum: > 0
     fn area(&mut self, area: f32) -> &mut Self {
+        if area <= 0.0 {
+            self.add_validation_error("area", "Must be greater than 0")
+        }
         self.add_attribute("area", AttributeText::from(area))
     }
 
@@ -1219,10 +1248,12 @@ pub trait NodeAttributes<'a> {
         self.add_attribute("group", AttributeText::attr(group))
     }
 
-    // TODO: constrain
     /// Height of node, in inches.
     /// default: 0.5, minimum: 0.02
     fn height(&mut self, height: f32) -> &mut Self {
+        if height < 0.02 {
+            self.add_validation_error("height", "Must be greater than or equal to 0.02")
+        }
         self.add_attribute("height", AttributeText::from(height))
     }
 
@@ -1336,6 +1367,9 @@ pub trait NodeAttributes<'a> {
     /// Used only if rotate is not defined.
     /// Default: 0.0 and minimum: 360.0
     fn orientation(&mut self, orientation: f32) -> &mut Self {
+        if orientation < 0.0 || orientation > 360.0 {
+            self.add_validation_error("orientation", "Must be between 0 and 360")
+        }
         Attributes::orientation(self.get_attributes_mut(), orientation);
         self
     }
@@ -1383,7 +1417,6 @@ pub trait NodeAttributes<'a> {
         self.add_attribute("shape", AttributeText::from(shape))
     }
 
-    // TODO: constrain
     /// Print guide boxes in PostScript at the beginning of routesplines if
     /// showboxes=1, or at the end if showboxes=2.
     /// (Debugging, TB mode only!)
@@ -1398,11 +1431,13 @@ pub trait NodeAttributes<'a> {
         self.add_attribute("sides", AttributeText::from(sides))
     }
 
-    // TODO: constrain
     /// Skew factor for shape=polygon.
     /// Positive values skew top of polygon to right; negative to left.
     /// default: 0.0, minimum: -100.0
     fn skew(&mut self, skew: f32) -> &mut Self {
+        if skew < -100.0 {
+            self.add_validation_error("skew", "Must be greater than or equal to -100")
+        }
         self.add_attribute("skew", AttributeText::from(skew))
     }
 
@@ -1491,6 +1526,8 @@ pub trait NodeAttributes<'a> {
     ) -> &mut Self;
 
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>>;
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str);
 }
 
 pub trait EdgeAttributes<'a> {
@@ -1500,10 +1537,12 @@ pub trait EdgeAttributes<'a> {
         self.add_attribute("arrowhead", AttributeText::from(arrowhead))
     }
 
-    // TODO: constrain
     /// Multiplicative scale factor for arrowheads.
     /// default: 1.0, minimum: 0.0
     fn arrow_size(&mut self, arrow_size: f32) -> &mut Self {
+        if arrow_size < 0.0 {
+            self.add_validation_error("arrowsize", "Must be greater than or equal to 0")
+        }
         self.add_attribute("arrowsize", AttributeText::from(arrow_size))
     }
 
@@ -1685,7 +1724,6 @@ pub trait EdgeAttributes<'a> {
         self
     }
 
-    // TODO: constrain
     /// Determines, along with labeldistance, where the headlabel / taillabel are
     /// placed with respect to the head / tail in polar coordinates.
     /// The origin in the coordinate system is the point where the edge touches the node.
@@ -1694,6 +1732,12 @@ pub trait EdgeAttributes<'a> {
     /// with positive angles moving counterclockwise and negative angles moving clockwise.
     /// default: -25.0, minimum: -180.0
     fn label_angle(&mut self, label_angle: f32) -> &mut Self {
+        if label_angle < -180.0 {
+            self.add_validation_error(
+                "labelangle",
+                "Must be greater than or equal to -180",
+            )
+        }
         self.add_attribute("labelangle", AttributeText::from(label_angle))
     }
 
@@ -1721,11 +1765,16 @@ pub trait EdgeAttributes<'a> {
         self.add_attribute("labelfontname", AttributeText::attr(label_font_name))
     }
 
-    // TODO: constrains
     /// Font size, in points, used for headlabel and taillabel.
     /// If not set, defaults to edge’s fontsize.
     /// default: 14.0, minimum: 1.0
     fn label_font_size(&mut self, label_font_size: f32) -> &mut Self {
+        if label_font_size < 1.0 {
+            self.add_validation_error(
+                "labelfontsize",
+                "Must be greater than or equal to 1",
+            )
+        }
         self.add_attribute("labelfontsize", AttributeText::from(label_font_size))
     }
 
@@ -1801,7 +1850,6 @@ pub trait EdgeAttributes<'a> {
         self.add_attribute("sametail", AttributeText::quoted(same_tail))
     }
 
-    // TODO: constrain
     /// Print guide boxes in PostScript at the beginning of routesplines if showboxes=1, or at the
     /// end if showboxes=2.
     /// (Debugging, TB mode only!)
@@ -1876,12 +1924,11 @@ pub trait EdgeAttributes<'a> {
         self
     }
 
-    // TODO: contrain
     /// Weight of edge.
     /// The heavier the weight, the shorter, straighter and more vertical the edge is.
     /// default: 1, minimum: 0
     fn weight(&mut self, weight: u32) -> &mut Self {
-        self.add_attribute("weight", AttributeText::attr(weight.to_string()))
+        self.add_attribute("weight", AttributeText::from(weight))
     }
 
     /// External label for a node or edge.
@@ -1909,6 +1956,8 @@ pub trait EdgeAttributes<'a> {
     ) -> &mut Self;
 
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>>;
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str);
 }
 
 pub(crate) fn fmt_attributes(attributes: &IndexMap<String, AttributeText>) -> String {
@@ -1943,7 +1992,8 @@ mod test {
                 (Color::Named("yellow"), Some(0.3)),
                 (Color::Named("blue"), None),
             ])
-            .build();
+            .build()
+            .unwrap();
 
         assert_eq!(
             graph_attributes.get("fillcolor").unwrap().dot_string(),
diff --git a/src/dot.rs b/src/dot.rs
index 5c1dc29..a6828b7 100644
--- a/src/dot.rs
+++ b/src/dot.rs
@@ -6,6 +6,7 @@ use crate::attributes::{
 };
 use indexmap::IndexMap;
 use std::borrow::Cow;
+use std::borrow::Cow::Borrowed;
 use std::collections::HashMap;
 use std::fmt::{Debug, Display, Formatter};
 use std::io;
@@ -13,6 +14,14 @@ use std::io::prelude::*;
 
 static INDENT: &str = "    ";
 
+pub type ValidationResult<T> = std::result::Result<T, Vec<ValidationError>>;
+
+#[derive(Debug, PartialEq, Clone)]
+pub struct ValidationError {
+    pub message: Cow<'static, str>,
+    pub field: Cow<'static, str>,
+}
+
 pub trait DotString<'a> {
     fn dot_string(&self) -> Cow<'a, str>;
 }
@@ -301,6 +310,8 @@ pub struct GraphBuilder<'a> {
     edges: Vec<Edge<'a>>,
 
     comment: Option<String>,
+
+    errors: Vec<ValidationError>,
 }
 
 // TODO: id should be an escString
@@ -317,6 +328,7 @@ impl<'a> GraphBuilder<'a> {
             nodes: Vec::new(),
             edges: Vec::new(),
             comment: None,
+            errors: Vec::new(),
         }
     }
 
@@ -332,6 +344,7 @@ impl<'a> GraphBuilder<'a> {
             nodes: Vec::new(),
             edges: Vec::new(),
             comment: None,
+            errors: Vec::new(),
         }
     }
 
@@ -411,7 +424,14 @@ impl<'a> GraphBuilder<'a> {
         self
     }
 
-    pub fn build(&self) -> Graph<'a> {
+    pub fn build(&self) -> ValidationResult<Graph<'a>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
+        }
+        Ok(self.build_ignore_validation())
+    }
+
+    pub fn build_ignore_validation(&self) -> Graph<'a> {
         Graph {
             id: self.id.to_owned(),
             is_directed: self.is_directed,
@@ -479,6 +499,8 @@ pub struct SubGraphBuilder<'a> {
     nodes: Vec<Node<'a>>,
 
     edges: Vec<Edge<'a>>,
+
+    errors: Vec<ValidationError>,
 }
 
 // TODO: id should be an escString
@@ -492,6 +514,7 @@ impl<'a> SubGraphBuilder<'a> {
             sub_graphs: Vec::new(),
             nodes: Vec::new(),
             edges: Vec::new(),
+            errors: Vec::new(),
         }
     }
 
@@ -573,8 +596,12 @@ impl<'a> SubGraphBuilder<'a> {
         self
     }
 
-    pub fn build(&self) -> SubGraph<'a> {
-        SubGraph {
+    pub fn build(&self) -> ValidationResult<SubGraph<'a>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
+        }
+
+        Ok(SubGraph {
             id: self.id.to_owned(),
             graph_attributes: self.graph_attributes.clone(),
             node_attributes: self.node_attributes.clone(),
@@ -582,7 +609,7 @@ impl<'a> SubGraphBuilder<'a> {
             sub_graphs: self.sub_graphs.clone(),
             nodes: self.nodes.clone(), // TODO: is clone the only option here?
             edges: self.edges.clone(), // TODO: is clone the only option here?
-        }
+        })
     }
 }
 
@@ -614,6 +641,7 @@ impl<'a> DotString<'a> for Node<'a> {
 pub struct NodeBuilder<'a> {
     id: String,
     attributes: IndexMap<String, AttributeText<'a>>,
+    errors: Vec<ValidationError>,
 }
 
 impl<'a> NodeAttributes<'a> for NodeBuilder<'a> {
@@ -638,6 +666,13 @@ impl<'a> NodeAttributes<'a> for NodeBuilder<'a> {
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>> {
         &mut self.attributes
     }
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str) {
+        self.errors.push(ValidationError {
+            field: Borrowed(field),
+            message: Borrowed(message),
+        })
+    }
 }
 
 impl<'a> NodeBuilder<'a> {
@@ -645,10 +680,18 @@ impl<'a> NodeBuilder<'a> {
         Self {
             id,
             attributes: IndexMap::new(),
+            errors: Vec::new(),
+        }
+    }
+
+    pub fn build(&self) -> ValidationResult<Node<'a>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
         }
+        Ok(self.build_ignore_validation())
     }
 
-    pub fn build(&self) -> Node<'a> {
+    pub fn build_ignore_validation(&self) -> Node<'a> {
         Node {
             // TODO: are these to_owned and clones necessary?
             id: self.id.to_owned(),
@@ -699,6 +742,7 @@ pub struct EdgeBuilder<'a> {
     pub target: String,
     pub target_port_position: Option<PortPosition>,
     attributes: IndexMap<String, AttributeText<'a>>,
+    errors: Vec<ValidationError>,
 }
 
 impl<'a> EdgeAttributes<'a> for EdgeBuilder<'a> {
@@ -714,6 +758,13 @@ impl<'a> EdgeAttributes<'a> for EdgeBuilder<'a> {
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>> {
         &mut self.attributes
     }
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str) {
+        self.errors.push(ValidationError {
+            field: Borrowed(field),
+            message: Borrowed(message),
+        })
+    }
 }
 
 impl<'a> EdgeBuilder<'a> {
@@ -724,6 +775,7 @@ impl<'a> EdgeBuilder<'a> {
             source_port_position: None,
             target_port_position: None,
             attributes: IndexMap::new(),
+            errors: Vec::new(),
         }
     }
 
@@ -739,6 +791,7 @@ impl<'a> EdgeBuilder<'a> {
             source_port_position: Some(source_port_position),
             target_port_position: Some(target_port_position),
             attributes: IndexMap::new(),
+            errors: Vec::new(),
         }
     }
 
@@ -770,7 +823,14 @@ impl<'a> EdgeBuilder<'a> {
         self
     }
 
-    pub fn build(&self) -> Edge<'a> {
+    pub fn build(&self) -> ValidationResult<Edge<'a>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
+        }
+        Ok(self.build_ignore_validation())
+    }
+
+    pub fn build_ignore_validation(&self) -> Edge<'a> {
         Edge {
             // TODO: are these to_owned and clones necessary?
             source: self.source.to_owned(),
@@ -804,21 +864,37 @@ impl<'a> NodeAttributes<'a> for NodeAttributeStatementBuilder<'a> {
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>> {
         &mut self.attributes
     }
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str) {
+        self.errors.push(ValidationError {
+            field: Borrowed(field),
+            message: Borrowed(message),
+        })
+    }
 }
 
 // I'm not a huge fan of needing this builder but having a hard time getting around &mut without it
 pub struct NodeAttributeStatementBuilder<'a> {
     pub attributes: IndexMap<String, AttributeText<'a>>,
+    errors: Vec<ValidationError>,
 }
 
 impl<'a> NodeAttributeStatementBuilder<'a> {
     pub fn new() -> Self {
         Self {
             attributes: IndexMap::new(),
+            errors: Vec::new(),
         }
     }
 
-    pub fn build(&self) -> IndexMap<String, AttributeText<'a>> {
+    pub fn build(&self) -> ValidationResult<IndexMap<String, AttributeText<'a>>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
+        }
+        Ok(self.build_ignore_validation())
+    }
+
+    pub fn build_ignore_validation(&self) -> IndexMap<String, AttributeText<'a>> {
         self.attributes.clone()
     }
 }
@@ -836,21 +912,37 @@ impl<'a> EdgeAttributes<'a> for EdgeAttributeStatementBuilder<'a> {
     fn get_attributes_mut(&mut self) -> &mut IndexMap<String, AttributeText<'a>> {
         &mut self.attributes
     }
+
+    fn add_validation_error(&mut self, field: &'static str, message: &'static str) {
+        self.errors.push(ValidationError {
+            field: Borrowed(field),
+            message: Borrowed(message),
+        })
+    }
 }
 
 // I'm not a huge fan of needing this builder but having a hard time getting around &mut without it
 pub struct EdgeAttributeStatementBuilder<'a> {
     pub attributes: IndexMap<String, AttributeText<'a>>,
+    errors: Vec<ValidationError>,
 }
 
 impl<'a> EdgeAttributeStatementBuilder<'a> {
     pub fn new() -> Self {
         Self {
             attributes: IndexMap::new(),
+            errors: Vec::new(),
+        }
+    }
+
+    pub fn build(&self) -> ValidationResult<IndexMap<String, AttributeText<'a>>> {
+        if !self.errors.is_empty() {
+            return Err(self.errors.clone());
         }
+        Ok(self.build_ignore_validation())
     }
 
-    pub fn build(&self) -> IndexMap<String, AttributeText<'a>> {
+    pub fn build_ignore_validation(&self) -> IndexMap<String, AttributeText<'a>> {
         self.attributes.clone()
     }
 }
diff --git a/src/lib.rs b/src/lib.rs
index a21d8cb..98c17d2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -18,7 +18,8 @@
 //!         .add_node(Node::new("N0".to_string()))
 //!         .add_node(Node::new("N1".to_string()))
 //!         .add_edge(Edge::new("N0".to_string(), "N1".to_string()))
-//!         .build();
+//!         .build()
+//!         .unwrap();
 //!
 //! let dot = Dot { graph: g };
 //! println!("{}", dot);
@@ -42,7 +43,8 @@
 //!         .add_node(Node::new("N0".to_string()))
 //!         .add_node(Node::new("N1".to_string()))
 //!         .add_edge(Edge::new("N0".to_string(), "N1".to_string()))
-//!         .build();
+//!         .build()
+//!         .unwrap();
 //!
 //! let dot = Dot { graph: g };
 //! let mut writer= Vec::new();
@@ -78,18 +80,21 @@
 //!             .label("process #1".to_string())
 //!             .style(GraphStyle::Filled)
 //!             .color(Color::Named("lightgrey"))
-//!             .build(),
+//!             .build()
+//!             .unwrap(),
 //!     )
 //!     .add_node_attributes(
 //!         NodeAttributeStatementBuilder::new()
 //!             .style(NodeStyle::Filled)
 //!             .color(Color::Named("white"))
-//!             .build(),
+//!             .build()
+//!             .unwrap(),
 //!     )
 //!     .add_edge(Edge::new("a0".to_string(), "a1".to_string()))
 //!     .add_edge(Edge::new("a1".to_string(), "a2".to_string()))
 //!     .add_edge(Edge::new("a2".to_string(), "a3".to_string()))
-//!     .build();
+//!     .build()
+//!     .unwrap();
 //!
 //! let cluster_1 = SubGraphBuilder::new(Some("cluster_1".to_string()))
 //!     .add_graph_attributes(
@@ -97,28 +102,33 @@
 //!             .label("process #2".to_string())
 //!             .style(GraphStyle::Filled)
 //!             .color(Color::Named("blue"))
-//!             .build(),
+//!             .build()
+//!             .unwrap(),
 //!     )
 //!     .add_node_attributes(
 //!         NodeAttributeStatementBuilder::new()
 //!             .style(NodeStyle::Filled)
-//!             .build(),
+//!             .build()
+//!             .unwrap(),
 //!     )
 //!     .add_edge(Edge::new("b0".to_string(), "b1".to_string()))
 //!     .add_edge(Edge::new("b1".to_string(), "b2".to_string()))
 //!     .add_edge(Edge::new("b2".to_string(), "b3".to_string()))
-//!     .build();
+//!     .build()
+//!     .unwrap();
 //!
 //! let g = GraphBuilder::new_directed(Some("G".to_string()))
 //!     .add_node(
 //!         NodeBuilder::new("start".to_string())
 //!             .shape(Shape::Mdiamond)
-//!             .build(),
+//!             .build()
+//!             .unwrap(),
 //!     )
 //!     .add_node(
 //!         NodeBuilder::new("end".to_string())
 //!             .shape(Shape::Msquare)
-//!             .build(),
+//!             .build()
+//!             .unwrap(),
 //!     )
 //!     .add_sub_graph(cluster_0)
 //!     .add_sub_graph(cluster_1)
@@ -165,6 +175,7 @@
 
 pub mod attributes;
 pub mod dot;
+pub mod validation;
 
 #[doc(hidden)]
 pub use crate::dot::{
diff --git a/src/validation.rs b/src/validation.rs
new file mode 100644
index 0000000..f9990ae
--- /dev/null
+++ b/src/validation.rs
@@ -0,0 +1,9 @@
+use std::borrow::Cow;
+
+pub type ValidationResult<T> = std::result::Result<T, Vec<ValidationError>>;
+
+#[derive(Debug, PartialEq, Clone)]
+pub struct ValidationError {
+    pub message: Cow<'static, str>,
+    pub field: Cow<'static, str>,
+}
\ No newline at end of file