Add eval module

* geesh/eval.scm: New file.
* Makefile.am: Add it.

Makefile.am

@@ -40,6 +40,7 @@ MODULES =                                       \
   geesh/built-ins/echo.scm                      \
   geesh/built-ins.scm                           \
   geesh/environment.scm                         \
+  geesh/eval.scm                                \
   geesh/lexer.scm                               \
   geesh/parser.scm                              \
   geesh/repl.scm                                \

geesh/eval.scm

@@ -0,0 +1,45 @@
+;;; The Geesh Shell Interpreter
+;;; Copyright 2018 Timothy Sample <samplet@ngyro.com>
+;;;
+;;; This file is part of Geesh.
+;;;
+;;; Geesh is free software: you can redistribute it and/or modify
+;;; it under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation, either version 3 of the License, or
+;;; (at your option) any later version.
+;;;
+;;; Geesh is distributed in the hope that it will be useful,
+;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with Geesh.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (geesh eval)
+  #:use-module (geesh shell)
+  #:use-module (geesh word)
+  #:use-module (ice-9 match)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-26)
+  #:export (eval-sh))
+
+;;; Commentary:
+;;;
+;;; This module provides an interpreter for the Shell language.
+;;;
+;;; Code:
+
+(define* (eval-word env word #:key (split? #t) (rhs-tildes? #f))
+  ;; FIXME: Set the 'eval-cmd-sub' parameter.
+  (expand-word env word #:split? split? #:rhs-tildes? rhs-tildes?))
+
+(define (eval-sh env exp)
+  "Evaluate the Shell expression @var{exp} in the context of the Shell
+environment @var{env}."
+  (match exp
+    (('<sh-exec> words ..1)
+     (let ((args (append-map (cut eval-word env <>) words)))
+       (match args
+         ((name . args) (apply sh:exec env name args))
+         (() #f))))))