go: step2_eval basic functionality

2 files changed, 122 insertions, 0 deletions

diff --git a/go/src/step2_eval/step2_eval.go b/go/src/step2_eval/step2_eval.go
new file mode 100644
index 0000000..4aededa
--- /dev/null
+++ b/go/src/step2_eval/step2_eval.go
@@ -0,0 +1,107 @@
+package main
+
+import (
+    "bufio"
+    //"io"
+    "fmt"
+    "os"
+    "strings"
+    "errors"
+)
+
+import (
+    . "types"
+    "reader"
+    "printer"
+)
+
+// read
+func READ(str string) (MalType, error) {
+    return reader.Read_str(str)
+}
+
+// eval
+func eval_ast(ast MalType, env map[string]MalType) (MalType, error) {
+    //fmt.Printf("eval_ast: %#v\n", ast)
+    if Symbol_Q(ast) {
+        k := ast.(Symbol).Val
+        exp, ok := env[k]
+        if !ok { return nil, errors.New("'" + k + "' not found") }
+        return exp, nil
+    } else if List_Q(ast) {
+        lst := []MalType{}
+        for _, a := range ast.(List).Val {
+            exp, e := EVAL(a, env)
+            if e != nil { return nil, e }
+            lst = append(lst, exp)
+        }
+        return List{lst}, nil
+    } else {
+        return ast, nil
+    }
+}
+
+func EVAL(ast MalType, env map[string]MalType) (MalType, error) {
+    //fmt.Printf("EVAL: %#v\n", ast)
+    switch ast.(type) {
+    case List: // continue
+    default:   return eval_ast(ast, env)
+    }
+
+    el, e := eval_ast(ast, env)
+    if e != nil { return nil, e }
+    f, ok := el.(List).Val[0].(func([]MalType)(MalType, error))
+    if !ok { return nil, errors.New("attempt to call non-function") }
+    return f(el.(List).Val[1:])
+}
+
+// print
+func PRINT(exp MalType) (MalType, error) {
+    return printer.Pr_str(exp, true), nil
+}
+
+var repl_env = map[string]MalType{
+    "+": func(a []MalType) (MalType, error) {
+            return a[0].(int) + a[1].(int), nil
+         },
+    "-": func(a []MalType) (MalType, error) {
+            return a[0].(int) - a[1].(int), nil
+         },
+    "*": func(a []MalType) (MalType, error) {
+            return a[0].(int) * a[1].(int), nil
+         },
+    "/": func(a []MalType) (MalType, error) {
+            return a[0].(int) / a[1].(int), nil
+         },
+}
+
+// repl
+func rep(str string) (MalType, error) {
+    var exp MalType
+    var e error
+    if exp, e = READ(str); e != nil { return nil, e }
+    if exp, e = EVAL(exp, repl_env); e != nil { return nil, e }
+    if exp, e = PRINT(exp); e != nil { return nil, e }
+    return exp, nil
+}
+
+func main() {
+    rdr := bufio.NewReader(os.Stdin);
+    // repl loop
+    for {
+        fmt.Print("user> ");
+        text, err := rdr.ReadString('\n');
+        text = strings.TrimRight(text, "\n");
+        if (err != nil) {
+            return
+        }
+        var out MalType
+        var e error
+        if out, e = rep(text); e != nil {
+            if e.Error() == "<empty line>" { continue }
+            fmt.Printf("Error: %v\n", e)
+            continue
+        }
+        fmt.Printf("%v\n", out)
+    }
+}
diff --git a/go/src/types/types.go b/go/src/types/types.go
index a212725..e3d8163 100644
--- a/go/src/types/types.go
+++ b/go/src/types/types.go
@@ -19,3 +19,18 @@ type Vector struct {
     Val []MalType
 }
 
+// Symbols
+func Symbol_Q(obj MalType) bool {
+    switch obj.(type) {
+    case Symbol: return true
+    default:     return false
+    }
+}
+
+// Lists
+func List_Q(obj MalType) bool {
+    switch obj.(type) {
+    case List: return true
+    default:   return false
+    }
+}