Add gofmt check to make precommit.

Makefile

@@ -79,9 +79,9 @@ run: $(RUNNER)
 
 test: $(ACCEPT_PASS_FILES) $(COMPILER_PASS_FILES) $(COMPILER_EXPR_VISITOR_PASS_FILES) $(COMPILER_STMT_PASS_FILES) $(RUNTIME_PASS_FILE) $(STDLIB_PASS_FILES)
 
-precommit: cover lint test
+precommit: cover gofmt lint test
 
-.PHONY: all benchmarks clean cover golint lint precommit pylint run test
+.PHONY: all benchmarks clean cover gofmt golint lint precommit pylint run test
 
 # ------------------------------------------------------------------------------
 # grumpc compiler
@@ -141,6 +141,12 @@ cover: $(RUNTIME_COVER_FILE) $(TOOL_BINS)
 	@bash -c 'comm -12 <(coverparse $< | sed "s/^grumpy/runtime/" | sort) <(git diff --dst-prefix= $(DIFF_COMMIT) | diffrange | sort)' | sort -t':' -k1,1 -k2n,2 | sed 's/$$/: missing coverage/' | tee errors.err
 	@test ! -s errors.err
 
+build/gofmt.diff: $(wildcard runtime/*.go)
+	@gofmt -d $^ > $@
+
+gofmt: build/gofmt.diff
+	@if [ -s $< ]; then echo 'gofmt found errors, run: gofmt -w $(ROOT_DIR)/runtime/*.go'; false; fi
+
 $(GOLINT_BIN):
 	@go get -u github.com/golang/lint/golint