prebirth: Initial function map

The Prebirth JS compiler is getting much more sophisticated than I had
hoped, but I need something to work with here.  This starts to get us
in a good spot.

There are other conveniences I will want.  More to come.

* build-aux/bootstrap/prebirth.js
  (Compiler#constructor): Add method, accept fnmap.
  (Compiler#_sexpToEs): Use fnmap as needed.
  (fnmap): Add initial map.
  Instantiate `Compiler' with fnmap.

build-aux/bootstrap/prebirth.js

@@ -300,6 +300,21 @@ class Parser
  */
 class Compiler
 {
+    /**
+     * Initialize with function map
+     *
+     * The function map will be used to map certain functions into other
+     * names or forms.  For example, `js:console' may map to `console.log'
+     * and `if' to an `if' statement+expression.
+     *
+     * @param {Object} fnmap function map
+     */
+    constructor( fnmap )
+    {
+        this._fnmap = fnmap;
+    }
+
+
     /**
      * Compile AST into ECMAScript
      *
@@ -484,10 +499,22 @@ class Compiler
         // simple function application (fn ...args)
         const [ { value: fn }, ...args ] = t;
 
+        const mapentry = this._fnmap[ fn ];
+
+        // if the fnmap contains a function entry, then it will handle the
+        // remaining processing
+        if ( mapentry && ( typeof mapentry === 'function' ) ) {
+            return mapentry(
+                args,
+                this._sexpToEs.bind( this ),
+                this._bodyToEs.bind( this )
+            );
+        }
+
         // convert all remaining symbols (after the symbol representing the
         // function application) into arguments by parsing their sexps or
         // scalar values
-        const idfn   = this._idFromName( fn, true );
+        const idfn   = mapentry || this._idFromName( fn, true );
         const argstr = args.map( arg => this._sexpToEs( arg ) ).join( ", " );
 
         // final function application
@@ -496,6 +523,54 @@ class Compiler
 }
 
 
+/**
+ * Function aliases and special forms
+ *
+ * This map allows for a steady transition---items can be removed as they
+ * are written in Prebirth Lisp.  This should give us a sane (but still
+ * simple) environment with which we can start to self-host.
+ *
+ * String values are simple function aliases.  Function values take over the
+ * compilation of that function and allow for defining special forms.  The
+ * first argument to the function is the list of raw arguments (not yet
+ * compiled); the second argument is `Compiler#_sexpToEs'; and the third is
+ * `Compiler#bodyToEs'.
+ *
+ * These are by no means meant to be solid implementations.
+ *
+ * @type {Object}
+ */
+const fnmap = {
+    'js:console': 'console.log',
+
+    // simple if statement with optional else
+    'if': ( [ pred, t, f ], stoes ) =>
+        `if (${stoes(pred)}) {${stoes(t)};} ` +
+        ( ( f === undefined ) ? '' : `else {${stoes(f)};}` ),
+
+    // (let ((binding val) ...) ...body), compiled as a self-executing
+    // function which allows us to easily represent the return value of the
+    // entire expression while maintaining local scope
+    'let*': ( [ bindings, ...body ], stoes, btoes ) =>
+        "(function(){\n" +
+            bindings
+                .map( ([ x, val ]) => `    const ${stoes(x)} = ${stoes(val)};\n` )
+                .join( '' ) +
+            btoes( body ) + "\n" +
+        "    })()",
+
+    // similar to the above, but variables cannot reference one-another
+    'let': ( [ bindings, ...body ], stoes, btoes ) =>
+        "(function(" +
+            bindings.map( ([ x ]) => stoes( x ) ).join( ", " ) +
+        "){\n" +
+            btoes( body ) + "\n" +
+        "})(" +
+            bindings.map( ([ , val ]) => stoes( val ) ).join( ", " ) +
+        ")",
+};
+
+
 
 /*
  * Prebirth was originally intended to be run via the command line using
@@ -510,7 +585,7 @@ class Compiler
     }
 
     const p = new Parser();
-    const c = new Compiler();
+    const c = new Compiler( fnmap );
 
     const fs   = require( 'fs' );
     const src  = fs.readFileSync( '/dev/stdin' ).toString();