Types

Fenl operates on typed values. Fenl's type system describes several different kinds of values.

Every expression has an associated type. Since every expression produces a column of values (corresponding to the value at specific points in time), each expression can be thought of as a column of the given type.

Simple values such as the string "hello" or the integer 57 are scalar types. They correspond to a column containing values of the given type (or null).

Types may be combined to create records. Record fields may be scalar or nested record types. An expression producing a record type is a column that produces a value of the given record type or null at each point in time.

Scalars

Scalar types include booleans, numbers, strings, timestamps, durations and calendar intervals.

Records

Records allow combining multiple different types into a single value. Records are unnamed - any two records with the same set of fields and value types are considered equal. Fields within a record may have different types. Field names must start with a letter.

Type Coercion

Fenl implicitly coerces numeric types when different kinds of numbers are combined. For example adding a 64-bit signed integer value to a 32-bit floating point value produces a 64-point floating point value.

Type Promotion Rules

Type coercion will never produce an integer overflow or reduction in numeric precision. Such conversions may be explicitly specified using as.

The coercion rules can be summarized with the following rules:

1. Integers can be widened: i8 -> i16 -> i32 -> i64.
2. Unsigned integers can be widened: u8 -> u16 -> u32 -> u64.
3. Floating point numbers can be widened: f16 -> f32 -> f64.
4. Unsigned integers can be promoted to the next wider integer u8 -> i16, u16 -> i32, u32 -> i64.
5. All numbers may be converted to f64.
6. Strings may be implicitly converted to timestamps by attempting to parse them as RFC3339 values. The timestamp will be null for strings that don't successfully parse.

Numeric Type Coercion Table

When two numbers are used, Fenl attempts to promote them to a compatible type as the smallest type that both types may be converted to.
The following table shows the result of this promotion for pairs of numeric types.

ℹ️

Coercion to Floating-Point

Note that when u64 is combined with a signed type the result is f64. This is the only case where an operation between integers produces a floating-point value.

Functions and Signatures

Every function in Fenl has a type signature. For example, count(input: any, window: window = null) -> u32. This tells us many things about the function:

1. It takes two arguments input and window.
2. The first argument can be of any type (scalar or record).
3. The second argument must be a type of window (the result of a window function such as since or sliding).
4. The second argument (window) is optional, and provides a default value of null.
5. The result is a u32.

Parameters without default values are required. Required arguments may be provided by position or keyword. One required argument may be omitted, in which case it is implicitly $input. This allows for use of functions with the | (pipe) syntax. For instance, TableFoo | count() is treated as TableFoo | count($input) which is the same as TableFoo | count(input=$input, window=null).

Optional Parameters

Parameters with default values in the signature are optional. Arguments for optional parameters must be keyword arguments. For example count(window = since(...)) but not count(since(...)).

Type Constraints

When a type constraint (such as any) appears in a signature, all occurrences of that must be the same type. Type coercion is applied as necessary to make all of the arguments for that constraint compatible.

Additionally, each type constraint imposes restrictions on the types that are valid for arguments with that constraint, as shown in the table below.