lean2/src/kernel/instantiate.cpp

/*
Copyright (c) 2013 Microsoft Corporation. All rights reserved.
Released under Apache 2.0 license as described in the file LICENSE.

Author: Leonardo de Moura
*/
#include <algorithm>
#include "kernel/free_vars.h"
#include "kernel/replace_fn.h"
#include "kernel/metavar.h"

namespace lean {
expr instantiate_with_closed(expr const & a, unsigned n, expr const * s) {
    lean_assert(std::all_of(s, s+n, closed));

    auto f = [=](expr const & m, unsigned offset) -> expr {
        if (is_var(m)) {
            unsigned vidx = var_idx(m);
            if (vidx >= offset) {
                if (vidx < offset + n)
                    return s[n - (vidx - offset) - 1];
                else
                    return mk_var(vidx - n);
            } else {
                return m;
            }
        } else if (is_metavar(m)) {
            expr r = m;
            for (unsigned i = 0; i < n; i++)
                r = add_inst(r, offset + n - i - 1, s[i]);
            return r;
        } else {
            return m;
        }
    };
    return replace_fn<decltype(f)>(f)(a);
}
expr instantiate(expr const & a, unsigned s, unsigned n, expr const * subst) {
    auto f = [=](expr const & m, unsigned offset) -> expr {
        if (is_var(m)) {
            unsigned vidx = var_idx(m);
            if (vidx >= offset + s) {
                if (vidx < offset + s + n)
                    return lift_free_vars(subst[n - (vidx - s - offset) - 1], offset);
                else
                    return mk_var(vidx - n);
            } else {
                return m;
            }
        } else if (is_metavar(m)) {
            expr r = m;
            for (unsigned i = 0; i < n; i++)
                r = add_inst(r, offset + s + n - i - 1, lift_free_vars(subst[i], offset + n - i - 1));
            return r;
        } else {
            return m;
        }
    };
    return replace_fn<decltype(f)>(f)(a);
}
expr instantiate(expr const & e, unsigned n, expr const * s) {
    return instantiate(e, 0, n, s);
}
expr instantiate(expr const & e, unsigned i, expr const & s) {
    return instantiate(e, i, 1, &s);
}

bool is_head_beta(expr const & t) {
    return is_app(t) && is_lambda(arg(t, 0));
}

expr apply_beta(expr f, unsigned num_args, expr const * args) {
    lean_assert(is_lambda(f));
    unsigned m = 1;
    while (is_lambda(abst_body(f)) && m < num_args) {
        f = abst_body(f);
        m++;
    }
    lean_assert(m <= num_args);
    expr r = instantiate(abst_body(f), m, args);
    if (m == num_args) {
        return r;
    } else {
        buffer<expr> new_args;
        new_args.push_back(r);
        for (; m < num_args; m++)
            new_args.push_back(args[m]);
        return mk_app(new_args);
    }
}

expr head_beta_reduce(expr const & t) {
    if (!is_head_beta(t)) {
        return t;
    } else {
        return apply_beta(arg(t, 0), num_args(t) - 1, &arg(t, 1));
    }
}

expr beta_reduce(expr t) {
    auto f = [=](expr const & m, unsigned) -> expr {
        if (is_head_beta(m))
            return head_beta_reduce(m);
        else
            return m;
    };
    while (true) {
        expr new_t = replace_fn<decltype(f)>(f)(t);
        if (new_t == t)
            return new_t;
        else
            t = new_t;
    }
}
}