The value module provides a set of type-checking predicates and the identity passthrough function. These predicates return a boolean and accept any value, making them the primary way to branch on value type inside match or cond expressions.
Predicates
value/function?
Returns true when the value is a function.
["value/function?", "myFn"]
true when "myFn" resolves to a function in the environment.
value/number?
Returns true when the value is a number.
Result: true
value/string?
Returns true when the value is a string.
["value/string?", "lion"]
true
value/boolean?
Returns true when the value is a boolean.
Result: true
value/object?
Returns true when the value is an object and not null.
["value/object?", {"a": 1}]
true
value/array?
Returns true when the value is an array.
["value/array?", ["array/make", 1, 2]]
true
value/null?
Returns true when the value is exactly null.
Result: true
Host-only predicates
Two additional predicates are available but are primarily useful when Lion programs work with host values injected through the environment:
value/undefined? — returns true when the value is undefined. JavaScript APIs sometimes return undefined where Lion programs expect null.value/symbol? — returns true when the value is a JavaScript Symbol, which can appear in host objects or when using module.symbol sentinels.
value/identity
Returns its argument unchanged. This is useful as a no-op branch in match when you want to pass the matched value through without transformation.
Result: 42
Using predicates with match
Predicates are the natural arguments to match branches. Each branch pairs a predicate with a handler lambda:
["match",
"value",
["value/number?", ["lambda", ["x"], ["number/add", "x", 1]]],
["value/string?", ["lambda", ["x"], ["string/concat", "x", "!"]]],
["lambda", ["x"], "x"]]
Use value/identity as the handler when you want a branch to pass the value through unmodified. It is more explicit than an inline ["lambda", ["x"], "x"] and signals intent clearly.