MIST core syntax

This file explain the basic syntax and features of MIST Language.


Any line starting with # is a comment

Data types and asignations

In MIST there are 3 basic data types: booleans, integers and strings. In addition, exists complex data structures: Dictionaries and List. The asignation operator is “=”


Mist have suport for booleans: True and False You can use NOT and AND function to create complex boolean expressions.

foo = True
bar = False

foobar = AND(NOT(foo, NOT(bar))


Mist have support for integers numbers. For now, there are no arithmetic operators but it will be in the future.

foo = 43


Like other languages, strings are delimited by “

foo = "John"

In adition, you can use {} tu create formatted strings that include values from variables or function calls.

bar = "Hi {foo}"


Dictionaries are difined with JSON syntax and you can access it with []. It can contain values of different types.

mydict = { "foo": 4, "bar": "John"}
foo = mydict["foo"]


Lists are difined with JSON syntax and you can access it with [index]. It can contain values of different types.

mylist = ["foo", "bar"]
foo = mylist[0]

Variables and Parameters

Use de operator $ to access environments variables.


Use de operator % to access to command line parameters.

```bash example.mist print(%p1)

>mist run example.mist p1=hello


The syntax of conditional expressions is:

if (boolean expresion) { commands… } elsif (boolean expresion) { commands… } else { commands… }

See the following example:

foo = False
bar = True

if (foo) {
} elsif (bar) {
} else {

The result of the code is “B”


MIST follow an structured paradign similar to C Language. So the basic structure item is the function.

You can define a function with the following syntax:

function functionName (param1name, param2name, ...) [=> out1, out2...] {
    return value

NOTE: Be aware that out1 and out2 is refered to streams and not to the return value. This will be explained in the Streams section.


function salute(name) {
    return "Hello {name}"

You can invoke a function with positional parameters or with named parameters. For example, invoking previous defined function:


Variable Scopes

Variables defined outside a function can be considered global variables because they are visible inside other funcion as long as there are no other variable defined with the same name inside the function.

Variables defined inside a funcion are only visibles inside the function.

global = "Global"

function test() {
  local = "Local"
  # global exists here
  # local exists here

# global exists here
# local NOT exists here

function test2(global) {
  # global exists here but is not the same. It is a local variable with the same name that hide the real global

Include and Import

For organizing the code. Mist provides the directives include and import

Include files

include is used to include .mist files in your code. The result is exactly like copy the content of the specified file in your own file. Example:

include "./lib/module.mist"
include "module.mist"
include "https://github.com/BBVA/mist/raw/master/mist/catalog/festin.mist"

The first line include the local file module.mist at lib directory The second line include module.mist from current directory or from local catalog directory. The third line include the remote file festin.mist

Import files

MIST is implemented in Python. And you can import Python functions and work with them in MIST. import directive imports all the funcions defined in a Python file. Example:

python file ./test/mist_files/mylib.py
def myPrint(s: str, stack:list=None, commands:list=None):
file myProgram.mist
import "./test/mist_files/mylib.py"

Note that Python functions receives 2 additional parameters: stack and commands. You can use those parameters to implement advanced functionality. The import directive can also import remote files. Example:

file myProgram.mist
import "https://raw.githubusercontent.com/BBVA/mist/master/test/mist_files/mylib.py"


Source streams

In order to comunicate funtion MIST implements streams. When running a function you can specify a source stream to replace one parameter using the following syntax:

myfunction(:mystream,[param2, param3,...])


:mystream => myfunction([param2, param3,...])


mystream => myfunction([param2, param3,...])

Note that with arrow notation the streams is always mapped to first parameter of the function.

When a new element is inserted in the stream, the function myfunction will be asyncronous invoked using the element of the stream as first parameter. The other parameters will be the same on eachh invocation.

No more than one instance of myfunction will be running at the same time. If there are more elements in the streams they will wait in the stream using FIFO strategy.

You can also use the arrow notation to invoke a function.

(value or function) => myStream

In this case leftValue is a value (string, integer, dictionary…) or a variable. Example:

"hello" => print()

NOTE: You can only use one source stream when invoking a function.

Send operator

You can send data to an stream using the following syntax:

leftValue => myStream

In this case leftValue is a value (string, integer, dictionary…) or a variable. Example:

mystream => print()
"hello" => mystream
"John" => mystream

Function with target streams

A function can send data to one or several streams. When defining a function you can specify how many targets stream you have and an internal name.

function functionName (param1name, param2name, ...) [=> out1, out2...] {
    return value

out1 and out2 are the internal names of streams where the function will send data. Let us see this with an example:

The following code will print “foo” twice in the console:

function tee(i) => p {
    i => p

tee("foo") => myStream
myStream => print()

In contrast to source streams. You can can have more than one target streams when defining and invoking a function.

If we invoke a function that has output streams without specifying the real streams. The output data will be discarded.

Note than a function can return a value and also produce data to output streams.


We call chains to several functions connected with arrows.

The following examples will also print “foo” twice in the console:

tee("foo") => myStream => print()


tee("foo") => print()


"foo" => tee() => print()

Note that you can omit the name of the stream when connecting two functions. In this case, a hidden stream will be created automatically.

When a function produces to more than one stream. Only the first stream will be connected to the next function.

Reserved work: done

The reserver word “done” will stop the execution of a block of code (closure)

Used inside a function will work as a return without a value.

Used inside an if-else structure will stop the execution of the closure.

Object Method Call

Mist is implemented in Python. List, Strings, Dictionaries, etc. are Python objects. You can call method of any object by using the syntax:


For example

myText = "Hello World"

This will print “HELLO WORLD”

You can see the documentation about all these funcions in Python’s documentation

Call Python functions

You can call any Python function by using the prefix “py.”

Example: Call native Python “print” function:


In addition you can call python funcions from any available module.

Example: Call Python “loads” function from module “json”

r = py.json.loads('''{"a": 1}''')

Example: Call Python “quote” function from module “urllib.parse”

r = py.urllib.parse.quote("a=b")