Build D2 text from typed syntax nodes and parse values up front

README.md

@@ -149,6 +149,8 @@ See the [tests](/tests/test_d2) for more detailed usage examples.
 
 Names and labels that D2 cannot read bare (such as `a.b`, `x: y`, or the keyword `shape`) are quoted for you. Connection endpoints are written as given, so they can be dotted paths.
 Style values you set are always written, even `0` or `False`. `None` means unset.
+Colors are checked when the `Style` is built. Pass a CSS color name, `transparent`, a 3 or 6 digit hex code with or without `#`, a `linear-gradient(...)`, a `radial-gradient(...)`, a `${variable}` or a `d2.Color`. Hex codes are lowercased and written quoted, and anything else raises `ValueError`.
+Numbers that D2 limits are checked too: `opacity` 0 to 1, `stroke_width` 0 to 15, `stroke_dash` 0 to 10, `font_size` 8 to 100, and non-negative `border_radius`, grid gaps, `top` and `left`.
 
 ```python
 import d2

src/d2/__init__.py

@@ -1,4 +1,5 @@
 from ._render import D2Error, fmt, render, validate
+from ._syntax import Color
 from .class_shape import ClassField, ClassMethod, ClassShape
 from .connection import Arrowhead, ArrowheadShape, Connection, Direction
 from .diagram import Config, Diagram, Layer, Legend
@@ -12,6 +13,7 @@
     "ClassField",
     "ClassMethod",
     "ClassShape",
+    "Color",
     "Config",
     "Connection",
     "D2Error",

src/d2/_limits.py

@@ -0,0 +1,13 @@
+from typing import TypeVar
+
+Number = TypeVar("Number", bound=float)
+
+
+def bounded(
+    name: str, value: Number | None, low: float, high: float | None = None
+) -> Number | None:
+    if value is None or (value >= low and (high is None or value <= high)):
+        return value
+    if high is None:
+        raise ValueError(f"{name} must be at least {low}, got {value}")
+    raise ValueError(f"{name} must be between {low} and {high}, got {value}")

src/d2/_render.py

@@ -2,7 +2,7 @@
 import subprocess
 from typing import Literal
 
-from d2.diagram import Diagram
+from d2.diagram import Diagram, LayoutEngine
 
 RenderFormat = Literal["svg", "png", "pdf", "txt", "gif", "pptx"]
 
@@ -48,7 +48,7 @@ def render(
     diagram: Diagram | str,
     format: RenderFormat = "svg",
     *,
-    layout: Literal["dagre", "elk", "tala"] | None = None,
+    layout: LayoutEngine | None = None,
     theme: int | None = None,
     target: str = "",
     dark_theme: int | None = None,