Add replaceStrings primop

This is a generalisation of replaceChars in Nixpkgs.

doc/manual/expressions/builtins.xml

@@ -733,6 +733,23 @@ removeAttrs { x = 1; y = 2; z = 3; } [ "a" "x" "z" ]</programlisting>
   </varlistentry>
 
 
+  <varlistentry><term><function>builtins.replaceStrings</function>
+  <replaceable>from</replaceable> <replaceable>to</replaceable> <replaceable>s</replaceable></term>
+
+    <listitem><para>Given string <replaceable>s</replaceable>, replace
+    every occurrence of the strings in <replaceable>from</replaceable>
+    with the corresponding string in
+    <replaceable>to</replaceable>. For example,
+
+<programlisting>
+builtins.replaceStrings ["oo" "a"] ["a" "i"] "foobar"
+</programlisting>
+
+    evaluates to <literal>"fabir"</literal>.</para></listitem>
+
+  </varlistentry>
+
+
   <varlistentry><term><function>builtins.seq</function>
   <replaceable>e1</replaceable> <replaceable>e2</replaceable></term>
 

src/libexpr/primops.cc

@@ -1519,7 +1519,7 @@ static void prim_concatStringSep(EvalState & state, const Pos & pos, Value * * a
     PathSet context;
 
     auto sep = state.forceString(*args[0], context, pos);
-    state.forceList(*args[1]);
+    state.forceList(*args[1], pos);
 
     string res;
     res.reserve((args[1]->listSize() + 32) * sep.size());
@@ -1534,6 +1534,41 @@ static void prim_concatStringSep(EvalState & state, const Pos & pos, Value * * a
 }
 
 
+static void prim_replaceStrings(EvalState & state, const Pos & pos, Value * * args, Value & v)
+{
+    state.forceList(*args[0], pos);
+    state.forceList(*args[1], pos);
+    if (args[0]->listSize() != args[1]->listSize())
+        throw EvalError(format("‘from’ and ‘to’ arguments to ‘replaceStrings’ have different lengths, at %1%") % pos);
+
+    Strings from;
+    for (unsigned int n = 0; n < args[0]->listSize(); ++n)
+        from.push_back(state.forceStringNoCtx(*args[0]->listElems()[n], pos));
+
+    Strings to;
+    for (unsigned int n = 0; n < args[1]->listSize(); ++n)
+        to.push_back(state.forceStringNoCtx(*args[1]->listElems()[n], pos));
+
+    PathSet context;
+    auto s = state.forceString(*args[2], context, pos);
+
+    string res;
+    for (size_t p = 0; p < s.size(); ) {
+        bool found = false;
+        for (auto i = from.begin(), j = to.begin(); i != from.end(); ++i, ++j)
+            if (s.compare(p, i->size(), *i) == 0) {
+                found = true;
+                p += i->size();
+                res += *j;
+                break;
+            }
+        if (!found) res += s[p++];
+    }
+
+    mkString(v, res, context);
+}
+
+
 /*************************************************************
  * Versions
  *************************************************************/
@@ -1741,6 +1776,7 @@ void EvalState::createBaseEnv()
     addPrimOp("__hashString", 2, prim_hashString);
     addPrimOp("__match", 2, prim_match);
     addPrimOp("__concatStringsSep", 2, prim_concatStringSep);
+    addPrimOp("__replaceStrings", 3, prim_replaceStrings);
 
     // Versions
     addPrimOp("__parseDrvName", 1, prim_parseDrvName);

tests/lang/eval-okay-replacestrings.exp

@@ -0,0 +1 @@
+[ "faabar" "fbar" "fubar" "faboor" "fubar" ]

tests/lang/eval-okay-replacestrings.nix

@@ -0,0 +1,8 @@
+with builtins;
+
+[ (replaceStrings ["o"] ["a"] "foobar")
+  (replaceStrings ["o"] [""] "foobar")
+  (replaceStrings ["oo"] ["u"] "foobar")
+  (replaceStrings ["oo" "a"] ["a" "oo"] "foobar")
+  (replaceStrings ["oo" "oo"] ["u" "i"] "foobar")
+]