Adding a function to BAli-Phy

Since BAli-Phy models are written in Haskell, you can define functions inside your own model file. However, some functions are used frequently enough to add them to BAli-Phy itself instead of including them separately in multiple different model files.

Adding a function to BAli-Phy involves adding it to one of the Haskell modules that comes with BAli-Phy so that it can be used from multiple different model files. To make the function visible from the command line as well, you will need to add a JSON file to the bindings directory that describes how to access the Haskell function.

Haskell modules

In order to add a function to BAli-Phy you need to add it to a module. For example, if you add the function to the module SModel.Nucleotides then you could access it by adding

import SModel.Nucleotides

to the beginning of your model file.

The Haskell file for the module SModel.Nucleotides is located at haskell/SModel/Nucleotides.hs. (See the haskell/ directory on github.) The module file begins with a module declaration:

module SModel.Nucleotides where

We can then write a function inside this module. For example, in the module SModel.Nucleotides, we have the function hky85:

hky85 k    pi a = gtr a (hky85_sym k a) pi

This function defines the hky85 model as a function of the transition-transversion ratio (k), the equilibrium frequencies (pi) and the DNA or RNA alphabet (a).

Creating a new module

Let’s create a new module called Bio.MyModule that contains a function for squaring numbers. First we create a new module at haskell/Bio/MyModule.hs. Module names must begin with an upper-case letter. After the first letter, lower-case letters, numbers, and _ are allowed.

Then we add the following to the file:

module Bio.MyModule where

square x = x * x

After we rerun ninja install, the function square module will be accessible to model files that import Bio.MyModule.

Using a function from the command line

To make a Haskell function accessible from the command line, you must add a JSON file to the directory bindings/. For example, to add our a function square in the module Bio.MyModule to the command line interface, we could add a file called bindings/functions/square.json containing the text:

{
    "name": "square",
    "result_type": "Double",
    "import": ["Bio.MyModule"],
    "call": "square(x)",
    "args": [
        {
            "arg_name": "x",
            "arg_type": "Double",
        }
    ]
}

• The name property determines the name that is visible on the command line.
• The result_type property specifies the type of object returned by the function.
• The import property gives a list of modules that need to be imported to find the function.
• The call property specifies which Haskell function to call, and which arguments to pass.
• The args array specifies the name and type for each argument.

When you reinstall BAli-Phy, this file will land in the bindings directory, and you will be able to run bali-phy help square.

See additional explanation (forthcoming) for writing help for functions.

See additional explanation on writing default arguments or priors for functions.