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|
use crate::sexp::{SExp, SExp::*, SLeaf::*, util::*};
impl SExp {
/// Evaluates the s-expression one step.
///
/// **Integer operations**
///
/// Addition, subtraction, multiplication and division
/// are supported.
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Add, scons(1, scons(1, Nil))).step(),
/// Ok(Atom(Int(2)))
/// );
///
/// assert_eq!(
/// scons(Sub, scons(1, scons(2, Nil))).step(),
/// Ok(Atom(Int(-1)))
/// );
///
/// assert_eq!(
/// scons(Mul, scons(2, scons(3, Nil))).step(),
/// Ok(Atom(Int(6)))
/// );
///
/// assert_eq!(
/// scons(Div, scons(6, scons(2, Nil))).step(),
/// Ok(Atom(Int(3)))
/// );
/// ```
///
/// Division truncates the decimals.
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Div, scons(5, scons(2, Nil))).step(),
/// Ok(Atom(Int(2)))
/// );
/// ```
///
/// Also, addition and multiplication can take more than two arguments:
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Add, scons(1, scons(2, scons(3, Nil)))).step(),
/// Ok(Atom(Int(6)))
/// );
///
/// assert_eq!(
/// scons(Mul, scons(1, scons(2, scons(3, Nil)))).step(),
/// Ok(Atom(Int(6)))
/// );
/// ```
///
/// Here's an example of a bit more complicated expression
/// from wikipedias article on s-expressions.
/// ```rust
/// use melisp::sexp::{SExp::*, SLeaf::*, util::*};
///
/// fn main() {
/// let exp = scons(
/// Mul,
/// scons(
/// 2,
/// scons(
/// scons(Add, scons(3, scons(4, Nil))),
/// Nil
/// )
/// )
/// );
///
/// let exp = exp.step().unwrap();
/// assert_eq!(exp, scons(Mul, scons(2, scons(7, Nil))));
///
/// let exp = exp.step().unwrap();
/// assert_eq!(exp, Atom(Int(14)));
/// }
///
/// ```
pub fn step(self) -> Result<Self, String> {
match self {
// List processing
// op not a value
// -----------------------
// (op x) -> (op.step() x)
SCons(op, l) if !(*op).is_value() => Ok(scons(op.step()?, l)),
// op value and a1 .. an values, and b0, b1 .. bn not values
// ---------------------------------------------------------
// (op a1 ... an b) -> (op a1 ... an b0.step() b1 .. bn)
SCons(op, l) if !(*l).consists_of_values() => {
fn inner(s: SExp) -> Result<SExp, String> {
match s {
SCons(a, b) if (*a).is_value() => Ok(scons(a, inner(*b)?)),
SCons(a, b) => Ok(scons(a.step()?, b)),
x => x.step()
}
}
Ok(scons(op, inner(*l)?))
},
// Arithmetic
// t1, ..., tn integers
// ------------------------------
// (+ t1 ... tn) -> t1 + ... + tn
SCons(op, l) if *op == Atom(Add) => l.into_vec()?
.iter()
.fold(
Ok(0),
|acc, el| {
match el {
Int(x) => acc.map(|v| x+v),
_ => Err(
"'+' should only be given integers".to_string()
)
}
}
).map(|x| Atom(Int(x))),
// t1, ..., tn integers
// ------------------------------
// (* t1 ... tn) -> t1 * ... * tn
SCons(op, l) if *op == Atom(Mul) => l.into_vec()?
.iter()
.fold(
Ok(1),
|acc, el| {
match el {
Int(x) => acc.map(|v| x*v),
_ => Err(
format!("'*' should only be given integers, found {:?}", el)
)
}
}
).map(|x| Atom(Int(x))),
// t1,t2 integers
// --------------------
// (- t1 t2) -> t1 - t2
SCons(op, l) if *op == Atom(Sub) => {
let ls = l.into_vec()?;
if ls.len() == 2 {
match (ls[0].clone(), ls[1].clone()) {
(Int(a), Int(b)) => Ok(Atom(Int(a - b))),
_ => Err("'-' should be only given integers".to_string())
}
} else {
Err(format!("'-' should be given 2 arguments, found {}", ls.len()))
}
},
// t1,t2 integers
// --------------------
// (/ t1 t2) -> t1 / t2
SCons(op, l) if *op == Atom(Div) => {
let ls = l.into_vec()?;
if ls.len() == 2 {
match (ls[0].clone(), ls[1].clone()) {
(_, Int(0)) => Err("division by zero".to_string()),
(Int(a), Int(b)) => Ok(Atom(Int(a / b))),
_ => Err("'/' should be only given integers".to_string())
}
} else {
Err(format!("'/' should be given 2 arguments, found {}", ls.len()))
}
},
// t is value
// ----------
// t -> t
t if t.is_value() => Ok(t),
t => Err(format!("unimplemented: {:?}.step()", t)),
}
}
}
|
