Bioinformatics pipelines with Haskell and Shake
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LICENSE Cabalise Feb 3, 2017
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README.md

Bioshake: Bioinformatics pipelining with Shake

Bioshake is a bioinformatics workflow tool extending the Shake build system.

Pipelines are defined in terms of stages that connect together into simple streams. Here's an example pipeline:

Input :-> align :-> mappedOnly :-> sortBam :-> deDup :-> out ["aligned.bam"]

Each stage takes its input from the previous stage and generates output for the stage that follows. Some features of Bioshake pipelines are

  • There is no need to mention files between the stages: intermediate files are automatically named. The out stage simply copies its input to explicitly named files.
  • Each stage has strong type guarantees. For example, if a stage requires a sorted BAM file, it will be a type error to construct a pipeline that feeds it an unsorted BAM file. This allows many pipeline errors to be caught at compile time rather than runtime.

Quickstart

This repository includes Nix expressions for easy building. To use, install Nix, clone this repository, then a shell containing the necessary GHC environment for building Bioshake can be dropped into with "nix-shell". To compile and execute the example pipeline, the following commands can be used:

nix-shell --command 'ghc -o examples/simple examples/simple.lhs -O'
cd examples
nix-shell -p bwa samtools platypus --command "./simple *.fq"

How to use

Begin with something like this:

main = bioshake threads shakeOptions $ do ...

This initialises bioshake with a certain number of threads and with some Shake options.

The (simplified) type signature of the bioshake function is

bioshake :: Int -> ShakeOptions -> Rules () -> IO ()

and so the do clause corresponds to a Shake Rules () object; Bioshake pipelines are compiled down to Shake Rules () and so can be partially interleaved with them. (In Shake, a Rules a object is a (collection of) build rules that return data of type a.)

Within the Rules () block, a pipeline looks like

  compileRules $ do
    compile $ 
      Input :-> align :-> mappedOnly :-> sortBam :-> deDup :-> out ["aligned.bam"]
      
    ...

The compileRules and compile functions have the (again simplified) types

compileRules :: Compiler () -> Rules ()
compile :: Compilable a => a -> Compiler () 

So compile takes Compilable things (the pipeline in this case) and makes something suitable for compileRules, which generates the actual shake Rules.

A Bioshake pipeline must instantiate Compilable, but pipelines (and stages) are not themselves Compilers. Unlike vanilla Shake -- in which each stage of a build is represented by an object of type Rules a -- in Bioshake, there is no concrete datatype representing a stage; rather, each stage is the only inhabitant of its type, and the whole functionality of a stage is determined by the typeclasses that this type instantiates. More on this below.

Direct execution

In our pipeline, the stages align, mappedOnly, sortBam, and deDup come from different modules such as Bioshake.BWA and Bioshake.Samtools, or their cluster equivalents Bioshake.Cluster.BWA and Bioshake.Cluster.Samtools. Each module in bioshake represents a tool, such as an aligner; for example, Bioshake.BWA is the module that defines alignment stages using the BWA aligner. When a pipeline is executed using this module, the tool is directly invoked on the machine that is executing bioshake. Many of these tools are multithreaded, and thus you need to declare how many threads to use by default:

instance Default Threads where def = Threads threads

This is overridable for specific stages using the $~ operator. For example, if we wanted to restrict the above pipeline to only 5 threads during alignment we could write:

Input :-> align $~ Threads 5 :-> mappedOnly :-> sortBam :-> deDup :-> out ["aligned.bam"]

Note that bioshake will not overcommit resources: the first argument to bioshake defines the maximum number of concurrent threads at any given time.

Cluster submission

Stages can be submitted to a cluster instead of executing directly. In this case, the Bioshake.Cluster.* modules are used. Continuing the BWA example, to submit BWA jobs to a cluster instead of executing directly you would import Bioshake.Cluster.BWA. In this case, the default cluster configuration must be defined:

instance Default Config where def = Config [Queue "somequeue", Mem (gb 20),
CPUs 42]

There are a several configuration options that can be defined:

  1. Mem: the maximum memory used by the stage
  2. CPUs: the maximum number of CPUs
  3. Walltime: maximum walltime allowed
  4. Queue: the queue to submit to
  5. Module: a module to load before execution

Writing your own stages

As mentioned above, in ordinary Shake, each stage of a build is represented by an object of type Rules a. By contrast, in Bioshake, each stage inhabits its own type, and the functionality of a stage is determined by the typeclasses that its type instantiates. Many of these instances can be automatically generated by Template Haskell. The constructor :->, which is used to concatenate successive stages into a pipeline, is isomorphic to (,).

Here is an example showing how to create your own stage. We will create a stage called Magic that takes a BAM file as input and produces a VCF as output.

First, create a datatype to represent the stage:

data Magic c = Magic c deriving Show

The c type variable is a configuration type which is used by bioshake to represent threaded/cluster configuration. You can have other polymorphic types, but the last type must be the configuration type. Next, write a function that executes the tool:

buildMagic (Magic c) (paths -> [input]) [out] =
  run "/some/tool" ["-a", "with-flags"] [input] [out]

Here run is pretty much like a restricted version of shake's cmd. Now use template haskell to do the rest:

$(makeSingleTypes ''Magic [''IsVCF] [])
$(makeSingleThread ''Magic [''IsBAM] 'buildMagic)

The makeSingleTypes macro declares that Magic maps each input file to an output file (in this case buildMagic expects a single input and maps it to a single output). The first list in the template function declares properties about the output of the stage: the first must be a IsEXT tag, which declares the type of output and the extension. The second list is a bunch of transitive tags: if this tags hold for the input, then they will hold for the output too.

makeSingleThread declares how to build Magic things. The list is a bunch of tags that must hold for the input files, and the second parameter is just the function to build output files given inputs.

The Input stage

The first stage input is probably the most confusing part about getting a pipeline running. Stages generate some output that is fed to the next stage in the pipeline, hence the first stage has to "generate" some output files, which are just the input files on disk. We need some data structure to represent input files on the disk:

data Input = Input deriving Show

First is to declare the paths "output" from the stage. As the input stage, we just return a list of input files:

instance Pathable Input where
  paths _ = ["sample_R1.fastq.gz", "sample_R2.fastq.gz"]

We've assumed the inputs here are paired end reads, hence there are two fastq files. This is not sufficient as we must declare our inputs have certain properties: it is compilable to shake rules, that the reads are from paired end sequencing, that they are fastq files, and furthermore that they should be referenced against some genome:

instance Compilable Input
instance PairedEnd Input
instance IsFastQ Input
instance Referenced In where
  getRef _ = "/path/to/hg38.fa"
  name _ = "hg38"

Further documentation

There is an example pipeline in the repository and haddock documentation at https://papenfusslab.github.io/bioshake/.