Provide some help for migrating "curve"

src/style-spec/expression/definitions/curve.js

@@ -0,0 +1,26 @@
+// @flow
+
+import type { Expression } from '../expression';
+import type ParsingContext from '../parsing_context';
+import type { Type } from '../types';
+
+class Curve implements Expression {
+    key: string;
+    type: Type;
+
+    static parse(args: Array<mixed>, context: ParsingContext) {
+        const [ , interpolation, input, ...rest] = args;
+        if ((interpolation: any)[0] === "step") {
+            return context.error(`"curve" has been replaced by "step" and "interpolate". Replace this expression with ${
+                JSON.stringify(["step", input, ...rest])}`, 0);
+        } else {
+            return context.error(`"curve" has been replaced by "step" and "interpolate". Replace this expression with ${
+                JSON.stringify(["interpolate", interpolation, input, ...rest])}`, 0);
+        }
+    }
+
+    evaluate() {}
+    eachChild() {}
+}
+
+module.exports = Curve;

src/style-spec/expression/definitions/index.js

@@ -25,6 +25,7 @@ const Coercion = require('./coercion');
 const At = require('./at');
 const Match = require('./match');
 const Case = require('./case');
+const Curve = require('./curve');
 const Step = require('./step');
 const Interpolate = require('./interpolate');
 const Coalesce = require('./coalesce');
@@ -47,6 +48,7 @@ const expressions: { [string]: Class<Expression> } = {
     'case': Case,
     'match': Match,
     'coalesce': Coalesce,
+    'curve': Curve,
     'step': Step,
     'interpolate': Interpolate
 };

test/integration/expression-tests/curve/interpolate/test.json

@@ -0,0 +1,15 @@
+{
+  "expression": ["curve", ["linear"], 0, 1, 2, 3, 4],
+  "inputs": [[{}, {}]],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [
+        {
+          "key": "[0]",
+          "error": "\"curve\" has been replaced by \"step\" and \"interpolate\". Replace this expression with [\"interpolate\",[\"linear\"],0,1,2,3,4]"
+        }
+      ]
+    }
+  }
+}

test/integration/expression-tests/curve/step/test.json

@@ -0,0 +1,15 @@
+{
+  "expression": ["curve", ["step"], 0, 1, 2, 3, 4, 5],
+  "inputs": [[{}, {}]],
+  "expected": {
+    "compiled": {
+      "result": "error",
+      "errors": [
+        {
+          "key": "[0]",
+          "error": "\"curve\" has been replaced by \"step\" and \"interpolate\". Replace this expression with [\"step\",0,1,2,3,4,5]"
+        }
+      ]
+    }
+  }
+}