let assert_tuple = func (tpl) => select tpl is "tuple", NULL, {
    false = fail "@ is not a tuple" % (tpl),
};

// return a list of the fields in a tuple.
let fields = module{
    tpl = NULL,
} => {
    let assert_tuple = import "std/tuples.ucg".assert_tuple;
    // First we check that mod.tpl is a tuple.
    assert_tuple(mod.tpl);
    
    let reducer = func (acc, field, value) =>  acc + [field];

    let result = reduce reducer [], (mod.tpl);
};

// return a list of the values in a tuple.
let values = module{
    tpl = NULL,
} => {
    let assert_tuple = import "std/tuples.ucg".assert_tuple;
    // First we check that mod.tpl is a tuple.
    assert_tuple(mod.tpl);
    
    let reducer = func (acc, field, value) =>  acc + [value];

    let result = reduce reducer [], (mod.tpl);
};

let iter = module{
    tpl = NULL,
} => {
    let assert_tuple = import "std/tuples.ucg".assert_tuple;
    // First we check that mod.tpl is a tuple.
    assert_tuple(mod.tpl);
    
    let reducer = func (acc, field, value) =>  acc + [[field, value]];
    
    let result = reduce reducer [], (mod.tpl);
};

let strip_nulls = module{
    tpl = NULL,
} => {
    let assert_tuple = import "std/tuples.ucg".assert_tuple;
    // First we check that mod.tpl is a tuple.
    assert_tuple(mod.tpl);
    
    let filterer = func (name, value) =>  value != NULL;

    let result = filter filterer (mod.tpl);
};

let has_fields = module{
    tpl = NULL,
    fields = [],
} => {
    let lib = import "std/tuples.ucg";
    // First we check that mod.tpl is a tuple.
    lib.assert_tuple(mod.tpl);

    let fs = lib.fields{tpl=mod.tpl}.result;

    let reducer = func (acc, f) => acc && (f in fs);

    let result = reduce reducer true, (mod.fields);
};

let field_type = module{
    tpl = NULL,
    field = NULL,
    type = NULL,
} => {
    let lib = import "std/tuples.ucg";
    // First we check that mod.tpl is a tuple.
    lib.assert_tuple(mod.tpl);

    // Next we assert that mod.field is a string.
    select mod.field is "str", NULL, {
        false = fail "@ is not a string" % (mod.field),
    };

    // and finally that mod.type is a string
    select mod.type is "str", NULL, {
        false = fail "@ is not a string" % (mod.type),
    };

    let has_field = lib.has_fields{tpl=mod.tpl, fields=[mod.field]}.result; 
    
    let it = lib.iter{tpl=mod.tpl}.result;
    
    let reducer = func (acc, l) => acc && (select l.0 == mod.field, NULL, {
        true = l.1 is mod.type, // if this is the field then we propagate if it's the right type.
        false = true, // if this isn't the field then we propagate true
    });

    let is_type = reduce reducer true, it;

    let result = has_field && is_type;
};