SHS/stdlib/control_flow.go

/*   SHS: Syntactically Homogeneous Shell
 *   Copyright (C) 2019  Aidan Hahn
 *
 *   This program 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.
 *
 *   This program 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 this program.  If not, see <https://www.gnu.org/licenses/>.
 */

package stdlib

import (
    "gitlab.com/whom/shs/ast"
    "gitlab.com/whom/shs/log"
)

/* eval N forms. return the last one
 *
 * Example:
 *     (progn (print "hello") (print "world") (+ 1 2))
 *     This example will print "hello world" to stdout and return 3
 */
func ShsProgn(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    var res *ast.Token
    for iter := in; iter != nil; iter = iter.Next {
        res = iter.Eval(ft, vt, false)
    }

    return res
}

/* return one evaluated form or another based on the boolean statement
 * arg 1 is a boolean cond, arg 2 is evaluated if the cond is true, arg 3 is evaluated if cond is not true
 * in total it takes 3 arguments
 *
 * Example:
 *    (if (eq (number "3") 3) (print "test passed") (print "test failed"))
 */
func ShsIf(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    cond := in
    t := cond.Next
    f := t.Next
    cond.Next = nil
    t.Next = nil

    cond = cond.Eval(ft, vt, false)
    if cond == nil || cond.Tag != ast.BOOL {
        log.Log(log.ERR, 
            "first argument to if must be a bool statement",
            "if")
        return nil
    }

    switch cond.Value() {
    case ast.TRUE:
        return t

    case ast.FALSE:
        return f

    default:
        log.Log(log.ERR,
            "improper bool!",
            "if")
        return nil
    }
}

/* continually eval n forms while element #1 evals to T
 * has rather progn like behavior in that it returns the result of the last form to be evaluated
 *
 * Example: 
 *     (export cond F)
 *     (while cond (export cond T) (print "will only be printed once") (+ 1 2))
 *     loop will iter one time, print "will only be printed once" and return 3
 */
func ShsWhile(in *ast.Token, vt ast.VarTable, ft ast.FuncTable) *ast.Token {
    cond := in
    forms := in.Next
    in.Next = nil
    var res *ast.Token

    eval := cond.Eval(ft, vt, false)
    if eval == nil || eval.Tag != ast.BOOL {
        log.Log(log.ERR,
            "first argument to while must be a bool statement",
            "while")
        return nil
    }

    // slight downside here: doesnt log when the wrong tag is set
    for eval.Tag == ast.BOOL && eval.Value() == ast.TRUE {
        // eval all forms
        for i := forms; i != nil; i = i.Next {
            res = i.Eval(ft, vt, false)
        }

        // retest eval
        eval = cond.Eval(ft, vt, false)
    }

    return res
}