Add builtin function ‘fromJSON’

Fixes #294.

doc/manual/builtins.xml

@@ -257,6 +257,22 @@ stdenv.mkDerivation {
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.fromJSON</function> <replaceable>e</replaceable></term>
+
+    <listitem><para>Convert a JSON string to a Nix
+    value. For example,
+
+<programlisting>
+builtins.fromJSON ''{"x": [1, 2, 3], "y": null}''
+</programlisting>
+
+    returns the value <literal>{ x = [ 1 2 3 ]; y = null;
+    }</literal>. Floating point numbers are not
+    supported.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.getAttr</function>
   <replaceable>s</replaceable> <replaceable>set</replaceable></term>
 
@@ -762,6 +778,7 @@ in foo</programlisting>
 
   </varlistentry>
 
+
   <varlistentry><term><function>builtins.toPath</function> <replaceable>s</replaceable></term>
 
     <listitem><para>Convert the string value

src/libexpr/json-to-value.cc

@@ -0,0 +1,144 @@
+#include "config.h"
+#include "json-to-value.hh"
+
+#include <cstring>
+
+namespace nix {
+
+
+static void skipWhitespace(const char * & s)
+{
+    while (*s == ' ' || *s == '\t' || *s == '\n' || *s == '\r') s++;
+}
+
+
+#if HAVE_BOEHMGC
+typedef std::vector<Value *, gc_allocator<Value *> > ValueVector;
+#else
+typedef std::vector<Value *> ValueVector;
+#endif
+
+
+static string parseJSONString(const char * & s)
+{
+    string res;
+    if (*s++ != '"') throw JSONParseError("expected JSON string");
+    while (*s != '"') {
+        if (!*s) throw JSONParseError("got end-of-string in JSON string");
+        if (*s == '\\') {
+            s++;
+            if (*s == '"') res += '"';
+            else if (*s == '\\') res += '\\';
+            else if (*s == '/') res += '/';
+            else if (*s == '/') res += '/';
+            else if (*s == 'b') res += '\b';
+            else if (*s == 'f') res += '\f';
+            else if (*s == 'n') res += '\n';
+            else if (*s == 'r') res += '\r';
+            else if (*s == 't') res += '\t';
+            else if (*s == 'u') throw JSONParseError("\\u characters in JSON strings are currently not supported");
+            else throw JSONParseError("invalid escaped character in JSON string");
+            s++;
+        } else
+            res += *s++;
+    }
+    s++;
+    return res;
+}
+
+
+static void parseJSON(EvalState & state, const char * & s, Value & v)
+{
+    skipWhitespace(s);
+
+    if (!*s) throw JSONParseError("expected JSON value");
+
+    if (*s == '[') {
+        s++;
+        ValueVector values;
+        values.reserve(128);
+        skipWhitespace(s);
+        while (1) {
+            if (values.empty() && *s == ']') break;
+            Value * v2 = state.allocValue();
+            parseJSON(state, s, *v2);
+            values.push_back(v2);
+            skipWhitespace(s);
+            if (*s == ']') break;
+            if (*s != ',') throw JSONParseError("expected `,' or `]' after JSON array element");
+            s++;
+        }
+        s++;
+        state.mkList(v, values.size());
+        for (size_t n = 0; n < values.size(); ++n)
+            v.list.elems[n] = values[n];
+    }
+
+    else if (*s == '{') {
+        s++;
+        state.mkAttrs(v, 1);
+        while (1) {
+            skipWhitespace(s);
+            if (v.attrs->empty() && *s == '}') break;
+            string name = parseJSONString(s);
+            skipWhitespace(s);
+            if (*s != ':') throw JSONParseError("expected `:' in JSON object");
+            s++;
+            Value * v2 = state.allocValue();
+            parseJSON(state, s, *v2);
+            v.attrs->push_back(Attr(state.symbols.create(name), v2));
+            skipWhitespace(s);
+            if (*s == '}') break;
+            if (*s != ',') throw JSONParseError("expected `,' or `}' after JSON member");
+            s++;
+        }
+        v.attrs->sort();
+        s++;
+    }
+
+    else if (*s == '"') {
+        mkString(v, parseJSONString(s));
+    }
+
+    else if (isdigit(*s) || *s == '-') {
+        bool neg = false;
+        if (*s == '-') {
+            neg = true;
+            if (!*++s) throw JSONParseError("unexpected end of JSON number");
+        }
+        NixInt n = 0;
+        // FIXME: detect overflow
+        while (isdigit(*s)) n = n * 10 + (*s++ - '0');
+        if (*s == '.' || *s == 'e') throw JSONParseError("floating point JSON numbers are not supported");
+        mkInt(v, neg ? -n : n);
+    }
+
+    else if (strncmp(s, "true", 4) == 0) {
+        s += 4;
+        mkBool(v, true);
+    }
+
+    else if (strncmp(s, "false", 5) == 0) {
+        s += 5;
+        mkBool(v, false);
+    }
+
+    else if (strncmp(s, "null", 4) == 0) {
+        s += 4;
+        mkNull(v);
+    }
+
+    else throw JSONParseError("unrecognised JSON value");
+}
+
+
+void parseJSON(EvalState & state, const string & s_, Value & v)
+{
+    const char * s = s_.c_str();
+    parseJSON(state, s, v);
+    skipWhitespace(s);
+    if (*s) throw JSONParseError(format("expected end-of-string while parsing JSON value: %1%") % s);
+}
+
+
+}

src/libexpr/json-to-value.hh

@@ -0,0 +1,13 @@
+#pragma once
+
+#include "eval.hh"
+
+#include <string>
+
+namespace nix {
+
+MakeError(JSONParseError, EvalError)
+
+void parseJSON(EvalState & state, const string & s, Value & v);
+
+}

src/libexpr/primops.cc

@@ -6,6 +6,7 @@
 #include "archive.hh"
 #include "value-to-xml.hh"
 #include "value-to-json.hh"
+#include "json-to-value.hh"
 #include "names.hh"
 #include "eval-inline.hh"
 
@@ -775,6 +776,14 @@ static void prim_toJSON(EvalState & state, const Pos & pos, Value * * args, Valu
 }
 
 
+/* Parse a JSON string to a value. */
+static void prim_fromJSON(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    string s = state.forceStringNoCtx(*args[0], pos);
+    parseJSON(state, s, v);
+}
+
+
 /* Store a string in the Nix store as a source file that can be used
    as an input by derivations. */
 static void prim_toFile(EvalState & state, const Pos & pos, Value * * args, Value & v)
@@ -1396,6 +1405,7 @@ void EvalState::createBaseEnv()
     // Creating files
     addPrimOp("__toXML", 1, prim_toXML);
     addPrimOp("__toJSON", 1, prim_toJSON);
+    addPrimOp("__fromJSON", 1, prim_fromJSON);
     addPrimOp("__toFile", 2, prim_toFile);
     addPrimOp("__filterSource", 2, prim_filterSource);
 

tests/lang/eval-okay-fromjson.exp

@@ -0,0 +1 @@
+true

tests/lang/eval-okay-fromjson.nix

@@ -0,0 +1,32 @@
+# RFC 7159, section 13.
+builtins.fromJSON
+  ''
+    {
+      "Image": {
+          "Width":  800,
+          "Height": 600,
+          "Title":  "View from 15th Floor",
+          "Thumbnail": {
+              "Url":    "http://www.example.com/image/481989943",
+              "Height": 125,
+              "Width":  100
+          },
+          "Animated" : false,
+          "IDs": [116, 943, 234, 38793, true  ,false,null, -100]
+        }
+    }
+  ''
+==
+  { Image =
+    { Width = 800;
+      Height = 600;
+      Title = "View from 15th Floor";
+      Thumbnail =
+        { Url = http://www.example.com/image/481989943;
+          Height = 125;
+          Width = 100;
+        };
+      Animated = false;
+      IDs = [ 116 943 234 38793 true false null (0-100) ];
+    };
+  }