Fold compound expressions with literal arguments into literals

src/style-spec/function/definitions/literal.js

@@ -4,7 +4,7 @@ const { Color, isValue, typeOf } = require('../values');
 
 import type { Type } from '../types';
 import type { Value }  from '../values';
-import type { Expression, ParsingContext }  from '../expression';
+import type { Expression, ParsingContext, CompilationContext }  from '../expression';
 
 const u2028 = /\u2028/g;
 const u2029 = /\u2029/g;
@@ -45,9 +45,19 @@ class Literal implements Expression {
         return new Literal(context.key, type, value);
     }
 
-    compile() {
-        const value = Literal.compile(this.value);
-        return typeof this.value === 'object' ?  `(${value})` : value;
+    compile(ctx: CompilationContext) {
+        let value;
+        if (this.type.kind === 'Color') {
+            value = `(new $this.Color(${(this.value: any).join(', ')}))`;
+        } else {
+            value = Literal.compile(this.value);
+        }
+
+        if (typeof this.value === 'object' && this.value !== null) {
+            return ctx.addVariable(value);
+        } else {
+            return value;
+        }
     }
 
     static compile(value: Value) {

src/style-spec/function/evaluation_context.js

@@ -62,6 +62,7 @@ function ensure(condition: any, message: string) {
 module.exports = () => ({
     types: types,
 
+    Color: Color, // used for compiling color literals
     ensure: ensure,
     error: (msg: string) => ensure(false, msg),
 

src/style-spec/function/parse_expression.js

@@ -1,5 +1,7 @@
 // @flow
 
+const Literal = require('./definitions/literal');
+const {CompilationContext} = require('./expression');
 import type {ParsingContext, Expression} from './expression';
 
 /**
@@ -55,6 +57,22 @@ function parseExpression(expr: mixed, context: ParsingContext): ?Expression {
                 }
             }
 
+            // If an expression's arguments are all literals, we can evaluate
+            // it immediately and replace it with a literal value in the
+            // parsed/compiled result.
+            if (isConstant(parsed)) {
+                const cc = new CompilationContext();
+                const ec = require('./evaluation_context')();
+                const compiled = cc.compileToFunction(parsed, ec);
+                try {
+                    const value = compiled({}, {});
+                    parsed = new Literal(parsed.key, parsed.type, value);
+                } catch (e) {
+                    context.error(e.message);
+                    return null;
+                }
+            }
+
             return parsed;
         }
 
@@ -68,4 +86,21 @@ function parseExpression(expr: mixed, context: ParsingContext): ?Expression {
     }
 }
 
+const nonConstantExpressions = [ 'error', 'get', 'has', 'properties', 'id', 'geometry-type', 'zoom' ];
+function isConstant(expression: Expression) {
+    const {CompoundExpression} = require('./compound_expression');
+    const Var = require('./definitions/var');
+    if (expression instanceof CompoundExpression && nonConstantExpressions.indexOf(expression.name) >= 0) {
+        return false;
+    } else if (expression instanceof Var) {
+        return false;
+    }
+
+    let constant = true;
+    expression.eachChild(arg => {
+        constant = constant && (arg instanceof Literal);
+    });
+    return constant;
+}
+
 module.exports = parseExpression;

test/integration/expression-tests/constant-folding/evaluation-error/test.json

@@ -0,0 +1,28 @@
+{
+  "expectExpressionType": {"kind": "Color"},
+  "expression": [
+    "curve",
+    ["step"],
+    ["get", "x"],
+    "black",
+    0,
+    "invalid",
+    10,
+    "blue"
+  ],
+  "inputs": [
+    [{}, {"properties": {"x": -1}}],
+    [{}, {"properties": {"x": 0}}],
+    [{}, {"properties": {"x": 5}}],
+    [{}, {"properties": {"x": 10}}],
+    [{}, {"properties": {"x": 11}}]
+  ],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [
+        {"key": "[5]", "error": "Could not parse color from value 'invalid'"}
+      ]
+    }
+  }
+}

test/integration/expression-tests/constant-folding/to-color/test.json

@@ -0,0 +1,16 @@
+{
+  "expectExpressionType": null,
+  "expression": ["to-color", "red"],
+  "inputs": [[{}, {}]],
+  "expected": {
+    "compiled": {
+      "result": "success",
+      "isFeatureConstant": true,
+      "isZoomConstant": true,
+      "type": "Color"
+    },
+    "outputs": [
+      [1, 0, 0, 1]
+    ]
+  }
+}