glslparser

An (incomplete) parser for GLSL shader programs, written in Haskell.

Final project submission for CMSC488b, Advanced Functional Programming.

Running

Try doing stack run <file_name_here>.

As an example, this repository contains two files shaders/test1.vert and shaders/test2.vert

stack run shaders/test1.vert, for example, will print a semi-pretty representation of the AST parsed from this file.

Functionality

Currently, this project only really just achieves parsing and AST printing. I did have higher hopes for my project submission, including pretty-printing (code formatting), optimization, and even limited execution at some point.

While I unfortunately just didn't have enough time (or skill) to make that happen, I'm still really happy with the result. I've gotten a taste of practically applying Haskell to one of its iconic domains, and have gotten insight in using things like Monads and Applicative. Parsing in Haskell using parser combinators is truly a joy compared to many other languages, and I've learned patterns I'll never be able to unsee!

Code Overview

First, the program input string is tokenized into discrete tokens, by the Lex module, of the form:

-- From Lex.hs
data Token = KW Keyword
           | Identifier String
           | IntConst Int
           | FloatConst Float
           | Op Operator
           | Semicolon
           | ScopeOpen
           | ScopeClose

Why? Couldn't we just parse the code directly? Yes! But, the GLSL specification is written in terms of discrete tokens, and it just makes everything a lot easier when I can follow the spec 1 to 1!

This is then parsed into a nice AST by the ParseTree module, with the structure of the AST being defined in the AST module. Abbreviated definition is as given:

-- From AST.hs
-- The ultimate structure, the parent of everything, representing a single source file
newtype TranslationUnit = TranslationUnit [Decl]

data FunctionArgument = FunctionArgument QualifiedType (Maybe String)

data Decl = FuncDecl QualifiedType String [FunctionArgument] CompoundStatement
          | VarsDecl QualifiedType String (Maybe Expr)

newtype CompoundStatement = CompoundStatement [Statement]

data Statement = EmptyStatement
               | DeclStatement Decl
               | ExprStatement Expr
               | IfStatement Expr Statement (Maybe Statement)
               | StatementList CompoundStatement

data Expr = VarExpr String
          | IntExpr Int
          | FloatExpr Float
          | UnaryExpr O.UnaryOp Expr
          | PostfixExpr Expr O.PostfixOp
          | BinExpr O.BinOp Expr Expr
          | AssignExpr O.Assignment Expr Expr
          | ConditionExpr Expr Expr Expr
          | FuncCallExpr String [Expr]

Limitations

As is likely plainly obvious, this parser supports a very limited subset of the GLSL language.

For example, we're missing:

The preprocessor
Type qualifiers! (const, uniform, in, out, centroid, etc...)
User-defined types (i.e. structs) and uniform blocks
Various different statements (while, do-while, switch/case, etc.)
Member accessors and vector swizzling

With time, I hope to add support for these!

Testing

A Quick Check property can be found in Lex.hs. It essentialy randomly generates a whole bunch of tokens, dumps it back to its string form, and checks to make sure they are tokenized in the exact same way.

10se1ucgo / glslparser