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Miscellaneous special forms

Inline documentation (doc)

Since JSON has no comments, PFA provides an inert special form for comments. It takes a string and always returns null.

{"doc": STRING}

where STRING is any string.


A simple example.

{"doc": "This is like those REM strings from BASIC."}

User-defined exceptions (error)

The following special form raises an exception. It has no return type.

{"error": STRING}


When combined with other types as a narrowest possible supertype, the "error" form is ignored. The following has type "int".

{"if": "condition",
 "then": 3,
 "else": {"error": "no good"}}

It can be used to let special forms return a non-null value without constructing unwanted unions.

Turning exceptions into missing values (try)

PFA has a "try" special form, but unlike most languages, it does not have a "catch" case. In PFA, "try" is used to turn exceptions into missing values (null). Its structure is




The EXPRESSION-OR-EXPRESSIONS is a single expression or a JSON array of expressions, and ARRAY-OF-STRINGS is a JSON array of simple strings (not expressions). If provided, the "filter" selects a subset of exception strings to catch.

The return type is a union of the expression's type and "null". This makes it applicable for any impute function.


The following gets the first element from an array or null if the array is empty.

{"try": {"a.head": "input"}}

If the "input" array had type {"type": "array", "items": "int"}, then the type of the above expression is ["null", "int"].

A traditional try-catch block can be formed by combining "try" with "ifnotnull":

{"ifnotnull": {"x": {"try": {"u.attempt": []}}},
 "then": {"u.workWithResult": "x"},
 "else": {"u.dealWithException": []}}

Log messages (log)

A PFA scoring engine is connected to an input source, and output sink, and optionally a log file. The nature of this log file is outside the scope of PFA, but if it exists, the following special form writes text to it.




where EXPRESSION-OR-EXPRESSIONS is a single expression or a JSON array of expressions, and NAME is a string (not an expression).

The output of each call to "log" is a line of text, space-delimited JSON representations of the provided expressions. The "namespace" is optional; the external system may use it to send logs to different files, prefix the line of text with a marker, or ignore it.

The return value of "log" is null.


The following writes the values of two variables, a number, and string to the logfile.

{"log": ["x", "y", 3.14, {"string": "literal string"}]}