Workflow Language


A module consists of:

  1. module header
  2. import statements
  3. type definitions
  4. external function declarations
  5. function definitions

in this order, for instance:

module ModuleB:0.1.0

import ModuleA:0.1.0 as A

type BType = A.Type

fun b : Text -> (BType, A.OtherType)

def x = A.y | b

All sections except the module header are optional.

Import Statements

An import statement includes the module to be imported (MyModule:0.1.0) and its alias (A). The alias is used to qualify types and functions imported from the module, e.g. A.y.

Type Definitions

Types are defined using the type keyword:

type PlantInfo = { petalLength : Double
                 , petalWidth : Double
                 , sepalLength : Double
                 , sepalWidth : Double
type PlantSpecies = Text

The left-hand side of a type declaration is the new type name; the right-hand side must be an existing type.

A type defined in one module can be used in other module by prefixing it with the module alias:

module ModuleA:0.0.1
type AText = Text
module ModuleB:0.0.1
import ModuleA:0.0.1 as A
type B = (A.AText, A.AText)

Function Declarations

This section declares the types of functions that are backed by containers. Functions are declared with the fun keyword:

fun gotham : MovieRecordImage -> MovieRecordImage

Function Definitions

Definitions bind function names (x) to expressions (A.y | b). They start with the def keyword:

def z = filter x

If a name is not prefixed by a module alias, it refers to a function defined in the current module.


Expressions combine already defined functions through the following operations:


A.y | A.z

Every value produced by A.y is passed to A.z.

The output type of A.y must match the input type of A.z.


concat A.y or A.y*

A.y must be a function that produces lists of values, in which case concat A.y is a function that “unpacks” the lists and yields the same values one by one.


filter A.y or A.y?

A.y must be a function that produces “optional” (potentially missing) values, in which case filter A.y is a function that filters out missing values.

Type application


Some functions (notably, most sources and sinks) can be specialized to multiple input or output types. This is done with type application: Sources.http<Text> specializes Sources.http to the type Text.

Parameter application

A.y { one = "...", two = "..." }.

Parameters are analogous to UNIX environment variables in the following ways:

  1. Parameters are inherited. E.g. in

    def y = x
    def z = y { foo = "bar" }
both functions x and y will have access to foo when z is called.
  1. Parameters can be overridden. E.g. in

    def y = x { foo = "baz" }
    def z = y { foo = "bar" }
y overrides the value of foo that is passed to x. Therefore, x will see the value of foo as baz, not bar.

Parameters are used to configure sources and sinks — for instance, to specify how to connect to a PostgreSQL database.

Parameters can also be used to configure user-defined modules. Inside a Python nstack method, the value of parameter foo can be accessed as self.args["foo"].


The workflow language supports line and block comments. Line comments start with // and extend until the end of line. Block comments are enclosed in /* and */ and cannot be nested.

EBNF grammar

The syntax is defined in EBNF (ISO/IEC 14977) in terms of tokens.

module = 'module', module name
       , {import}
       , {type}
       , {declaration}
       , {definition}
import = 'import', module name, 'as', module alias;
type = 'type', name, '=', ( type expression | sum type );
declaration = 'fun', name, ':', type expression,
                          '->', top-level type expression;
definition = 'def', name, '=', expression;
top-level type expression = type expression | 'Void';
type expression = type expression1
                | 'optional', type expression 1
type expression1 = tuple
                 | struct
                 | array
                 | qualified name;
tuple = '(', ')'
      | '(', type expression, ',', type expression,
       {',', type expression}, ')';
struct = '{', name, ':', type expression,
        {',', name, ':', type expression}, '}';
sum type = name, type expression1, '|', name, type expression1,
        {'|', name, type expression1};
expression = expression1, {'|', expression1};
expression1 = application, '*'
            | application, '?'
            | 'concat', application
            | 'filter', application
application = term [arguments];
arguments = '{', argument binding, {',', argument binding}, '}';
argument binding = name, '=', literal;
term = '(', expression, ')'
     | qualified name ['<', type, '>']