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use crate::sexp::{SExp, SExp::*, SLeaf::*, util::*};
impl SExp {
/// Performs substitution on the s-expression.
///
/// Replaces all free occurrences of variables
/// named `name` with the given `value`
/// in this s-expression.
/// ```rust
/// use myslip::sexp::{SExp::*, SLeaf::*, util::*};
///
/// assert_eq!(
/// scons(Add, scons(var("a"), var("b"))).subst("b", &scons(Sub, scons(2, 1))),
/// scons(Add, scons(var("a"), scons(Sub, scons(2, 1))))
/// );
/// ```
pub fn subst(self, name: &str, value: &SExp) -> SExp {
match self {
SCons(a, b) => {
if let Some((varname, _)) = a.clone().check_let() {
if &varname == name {
return SCons(a, b);
}
}
if scons(a.clone(), b.clone()).is_fun() {
let arglist = b.clone().parts()[0].clone();
let arglist = arglist.parts().into_iter().map(|exp| match exp {
Atom(Var(s)) => s,
_ => todo!(),
}).collect::<Vec<String>>();
if arglist.contains(&name.to_string()) {
return SCons(a, b);
}
}
if let Some((scrut, pavec)) = scons(a.clone(), b.clone())
.get_case_scrut_pats_and_arms()
{
let scrut = scrut.subst(name, value);
let mut res = vec![];
for (pat, arm) in pavec {
if pat
.get_vars_bound_by_pattern()
.contains(&name.to_string())
{
res.push(scons(pat, arm));
}
else
{
res.push(scons(pat, arm.subst(name, value)));
}
}
return SExp::back_to_case(scrut, res);
}
scons((*a).subst(name, value), (*b).subst(name, value))
},
Atom(Var(x)) if x == name => value.clone(),
t => t,
}
}
}
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