// The list of base types for a UCG value.
let base_types = [
    "int",
    "float",
    "str",
    "bool",
    "null",
    "tuple",
    "list",
    "func",
    "module",
];

// Computes the base type of a value.
let base_type_of = func (val) => reduce(func (acc, f) => select (acc.val is f), f, {
    true = acc{typ = f},
    false = acc,
}, {val=val, typ="null"}, base_types).typ;

// Turns any schema check module into a compile failure.
// The module must export the computed value as the result field.
let must = func (m, msg) => select m, fail msg, {
    true = m,
};

// Any does a boolean match against a set of allowed shapes for a type.
// This module uses the shape module below so the same rules for checking
// the types against the source value apply for each example in the list.
//
// This module does not do partial matches for the shapes.
let any = module {
    // The source value to check.
    val=NULL,
    // The set of allowed type shapes it can be.
    types=[],
} => (result) {
    let schema = mod.pkg();

    let reducer = func (acc, t) => acc{
        ok = acc.ok || (schema.shaped{val=acc.val, shape=t, partial=false}),
    };
    let any = func (val, types) => reduce(reducer, {ok=false, val=val}, types);

    let result = any(mod.val, mod.types).ok;
};

// Compares a value against an example schema value. compares the "shape" of the
// value to see if it matches. The base type must be the same as the base type
// of the shape. For tuples any field in the shape tuple must be present in the 
// source value and must be of the same base type and shape. This module will
// recurse into nested tuples.
// 
// Lists are must contain types from the list shape they are compared against.
// and empty list shape means the list val can have any types it wants inside.
//
// We do not check that functions or modules have the same argument lengths or types
// nor we check that they output the same types.
let shaped = module {
    // The source value to validate
    val = NULL,
    
    // The shape to validate it against.
    shape = NULL,
    
    // Whether partial matches are accepted.
    // When set to true then the source value can have
    // fields in tuples that are not present in the
    // shape it is compared with.
    partial = true,
} => (result) {
    let schema = mod.pkg();

    let simple_handler = func (val, shape) => val is schema.base_type_of(shape);

    let tuple_handler = func (acc, name, value) => acc{
        ok = select (name) in acc.shape, mod.partial, {
            true = schema.shaped{val=value, shape=acc.shape.(name)},
        },
    };

    let list_handler = func(acc, value) => acc{
        ok = false || schema.any{val=value, types=acc.shape},
    };
    
    let result =select schema.base_type_of(mod.val), false, {
        str = simple_handler(mod.val, mod.shape),
        int = simple_handler(mod.val, mod.shape),
        float = simple_handler(mod.val, mod.shape),
        bool = simple_handler(mod.val, mod.shape),
        null = simple_handler(mod.val, mod.shape), 
        tuple = (mod.shape is "tuple") && reduce(tuple_handler, {shape=mod.shape, ok=false}, mod.val).ok, 
        list = select mod.shape == [], true, {
            false = reduce(list_handler, {shape=mod.shape, ok=false}, mod.val).ok,
        },
        func = simple_handler(mod.val, mod.shape),
        module = simple_handler(mod.val, mod.shape),
    };
};