Add extra type checking in std.flatMap

Makefile

@@ -247,7 +247,7 @@ libjsonnet++.so.$(VERSION): $(LIB_CPP_OBJ)
 
 # std.jsonnet.h is a generated file so the first build will fail if it isn't
 # specified as an explicit dependency (after that, deps are known from compiler output)
-core/desugarer.cpp: core/std.jsonnet.h
+.makebuild/core/desugarer.cpp.o: core/std.jsonnet.h
 
 # Encode standard library for embedding in C
 .makebuild/stdlib/to_c_array: stdlib/to_c_array.cpp

stdlib/std.jsonnet

@@ -299,9 +299,19 @@ limitations under the License.
     if !std.isFunction(func) then
       error ('std.flatMap first param must be function, got ' + std.type(func))
     else if std.isArray(arr) then
-      std.flattenArrays(std.makeArray(std.length(arr), function(i) func(arr[i])))
+      std.flattenArrays(std.makeArray(
+        std.length(arr), function(i)
+          local o = func(arr[i]);
+          if std.isArray(o) then o else
+            error ('std.flatMap on arrays, provided function must return an array, got ' + std.type(o))
+      ))
     else if std.isString(arr) then
-      std.join('', std.makeArray(std.length(arr), function(i) func(arr[i])))
+      std.join('', std.makeArray(
+        std.length(arr), function(i)
+          local o = func(arr[i]);
+          if std.isString(o) then o else
+            error ('std.flatMap on strings, provided function must return a string, got ' + std.type(o))
+      ))
     else error ('std.flatMap second param must be array / string, got ' + std.type(arr)),
 
   join(sep, arr)::

test_suite/error.flatMap_array_typecheck.jsonnet

@@ -0,0 +1 @@
+std.flatMap(function(x) x, ['a', 'b', 'c'])

test_suite/error.flatMap_array_typecheck.jsonnet.golden

@@ -0,0 +1,11 @@
+RUNTIME ERROR: std.flatMap on arrays, provided function must return an array, got string
+	std.jsonnet:<stdlib_position_redacted>	thunk <array_element>
+	std.jsonnet:<stdlib_position_redacted>	thunk <b>
+	std.jsonnet:<stdlib_position_redacted>	function <func>
+	std.jsonnet:<stdlib_position_redacted>	thunk <running>
+	std.jsonnet:<stdlib_position_redacted>	function <aux>
+	std.jsonnet:<stdlib_position_redacted>	function <aux>
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	std.jsonnet:(302:7)-(307:9)	function <anonymous>
+	error.flatMap_array_typecheck.jsonnet:1:1-44	

test_suite/error.flatMap_seq_typecheck.jsonnet

@@ -0,0 +1 @@
+std.flatMap(function(x) [x], { a: 1, b: 2, c: 3 })

test_suite/error.flatMap_seq_typecheck.jsonnet.golden

@@ -0,0 +1,3 @@
+RUNTIME ERROR: std.flatMap second param must be array / string, got object
+	std.jsonnet:<stdlib_position_redacted>	function <anonymous>
+	error.flatMap_seq_typecheck.jsonnet:1:1-51	

test_suite/error.flatMap_string_typecheck.jsonnet

@@ -0,0 +1 @@
+std.flatMap(function(x) std.codepoint(x), 'abc')

test_suite/error.flatMap_string_typecheck.jsonnet.golden

@@ -0,0 +1,4 @@
+RUNTIME ERROR: std.flatMap on strings, provided function must return a string, got number
+	std.jsonnet:<stdlib_position_redacted>	thunk <array_element>
+	std.jsonnet:(309:7)-(314:9)	function <anonymous>
+	error.flatMap_string_typecheck.jsonnet:1:1-49