add piston nix pkg for brainfuck

runtimes/brainfuck.nix

@@ -0,0 +1,55 @@
+{pkgs, piston, ...}:
+let
+    # All other BF packages are either marked as broken in nix-pkgs
+    # or have missing functionality. Yabi isn't perfect either.
+    pkg = pkgs.haskellPackages.yabi;
+in piston.mkRuntime {
+    language = "brainfuck";
+    version = pkg.version;
+
+    aliases = [
+        "bf"
+        "yabi"
+    ];
+
+    run = ''
+        # Yabi produces messages when parsing the BF file that are sent to stderr
+        # Yabi will also break when trying to take more input at EOF
+        ${pkg}/bin/yabi "$1" 2> /dev/null
+    '';
+
+    tests = [
+        # stdout test
+        (piston.mkTest {
+            files = {
+                "test.bf" = ''-[--->+<]>------.----.'';
+            };
+            args = [];
+            stdin = "";
+            packages = [];
+            main = "test.bf";
+        })
+
+        # stdin test
+        (piston.mkTest {
+            files = {
+                "test.bf" = '',.,.'';
+            };
+            args = [];
+            stdin = "OK";
+            packages = [];
+            main = "test.bf";
+        })
+
+        # bf doesn't take args test
+        (piston.mkTest {
+            files = {
+                "test.bf" = ''-[--->+<]>------.----.'';
+            };
+            args = ["OK"];
+            stdin = "";
+            packages = [];
+            main = "test.bf";
+        })
+    ];
+}