Butler expression language

Butler registry supports a user-supplied expression for constraining input, output, or intermediate datasets that can appear in the generated QuantumGraph. This page describes the structure and syntax of the expression language.

The language grammar is defined in exprParser.parserYacc module, which is responsible for transforming string with user expression into a syntax tree with nodes represented by various classes defined in exprParser.exprTree module. Modules in exprParser package are considered butler/registry implementation details and are not exposed at the butler package level.

The gramma is based on standard SQL, it is a subset of SQL expression language that can appear in WHERE clause of standard SELECT statement with some extensions, e.g. range support for IN operator.

Expression structure

The expression is passed as a string to a registry methods that build a QuantumGraph typically from a command-line application such as pipetask. The string contains a single boolean expression which evaluates to true or false (if it is a valid expression). Expression can contain a bunch of standard logical operators, comparisons, literals, and identifiers which are references to registry objects.

A few words in expression grammar are reserved: AND, OR, NOT and IN. Reserved words are not case sensitive and can appear in either upper or lower case, or a mixture of both.

Language operator precedence rules are the same as for the other languages like C++ or Python. When in doubt use grouping operators (parentheses) for sub-expressions.

General note - the parser itself does not evaluate any expressions even if they consist of literals only, all evaluation happens in the SQL engine when registry runs the resulting SQL query.

Following sections describe each of the parts in detail.

Literals

The language supports these types of literals:

Strings

This is just a sequence of characters enclosed in single quotation marks. The parser itself fully supports Unicode, but some tools such as database drivers may have limited support for it, depending on environment or encoding chosen.

Numbers

Integer numbers are series of decimal numbers optionally preceded by minus sign. Parser does not support octal/hexadecimal numbers. Floating point numbers use standard notation with decimal point and/or exponent. For numbers parser passes a string representation of a number to downstream registry code to avoid possible rounding issues.

Range literals

This sort of literal is allowed inside IN expressions only. It consists of two integer literals separated by double dots and optionally followed by a colon and one more integer literal. Two integers define start and stop values for the range; both are inclusive values. The optional third integer defines stride value, which defaults to 1; it cannot be negative. Ranges are equivalent to a sequence of integers (but not to intervals of floats).

Examples of range literals:

  • 1..5 – equivalent to 1,2,3,4,5

  • 1..10:3 – equivalent to 1,4,7,10

  • -10..-1:2 – equivalent to -10,-8,-6,-4,-2

Identifiers

Identifiers represent values external to a parser, such as values stored in a database. The parser itself cannot define identifiers or their values; it is the responsibility of translation layer (registry) to map identifiers into something sensible. Like in most programming languages, an identifier starts with a letter or underscore followed by zero or more letters, underscores, or digits. Parser also supports dotted identifiers consisting of two simple identifiers separated by a dot. Identifiers are case-sensitive on parser side but individual database back-ends may have special rules about case sensitivity.

In current implementation simple identifiers are used by registry to represent dimensions, e.g. visit identifier is used to represent a value of visit dimension in registry database. Dotted identifiers are mapped to tables and columns in registry database, e.g. detector.raft can be used for accessing raft name (obviously dotted names need knowledge of database schema and how SQL query is built).

Unary arithmetic operators

Two unary operators + (plus) and - (minus) can be used in the expressions in front of (numeric) literals, identifiers, or other expressions which should evaluate to a numeric value.

Binary arithmetic operators

Language supports five arithmetic operators - + (add), - (subtract), * (multiply), / (divide), and % (modulo). Usual precedence rules apply to these operators. Operands for them can be anything that evaluates to a numeric value.

Comparison operators

Language supports set of regular comparison operators - =, !=, <, <=, >, >=. This can be used on operands that evaluate to a numeric values, for (in)equality operators operands can also be boolean expressions.

Note

The equality comparison operator is a single = like in SQL, not double == like in Python or C++.

IN operator

The IN operator (and NOT IN) are an expanded version of a regular SQL IN operator. Its general syntax looks like:

<expression> IN ( <literal1>[, <literal2>, ... ])
<expression> NOT IN ( <literal1>[, <literal2>, ... ])

where each item in the right hand side list is one of the supported literals. Unlike regular SQL IN operator the list cannot contain expressions, only literals. The extension to regular SQL IN is that literals can be range literals as defined above. It can also be a mixture of integer literals and range literals (language allows mixing of string literals and ranges but it may not make sense when translated to SQL).

For an example of range usage, these two expressions are equivalent:

visit IN (100, 110, 130..145:5)
visit in (100, 110, 130, 135, 140, 145)

as are these:

visit NOT IN (100, 110, 130..145:5)
visit Not In (100, 110, 130, 135, 140, 145)

Boolean operators

NOT is the standard unary boolean negation operator.

AND and OR are binary logical and/or operators.

All boolean operators can work on expressions which return boolean values.

Grouping operator

Parentheses should be used to change evaluation order (precedence) of sub-expressions in the full expression.

Examples

Few examples of valid expressions using some of the constructs:

visit > 100 AND visit < 200

visit IN (100..200) AND tract = 500

visit IN (100..200) AND visit NOT IN (159, 191) AND abstract_filter = 'i'

(visit = 100 OR visit = 101) AND exposure % 2 = 1