Write You a Haskell Stephen Diehl

let f x = x + 1
Let ”f” [] (Lam ”x” (Op Add [Var ”x”, Lit (LitInt 1)]))
Inference will generate a system of constraints which are solved via a process known as unification to yield
the type of the expression.
Int -> Int -> Int ~ a -> b
b ~ Int -> c
f :: Int -> Int
In some cases this type will be incorporated directly into the AST and the inference will transform the
frontend language into an explicitly typed core language.
Let ”f” []
(Lam ”x”
(TArr TInt TInt)
(App
(App
(Prim ”primAdd”) (Var ”x”))
(Lit (LitInt 1))))
Transformation
e type core representation is often suitable for evaluation, but quite often different intermediate representations
are more amenable to certain optimizations and make various semantic properties of the
language explicit. ese kind of intermediate forms will often attach information about free variables,
allocations, and usage information directly in the AST structure.
e most important form we will use is called the Spineless Tagless G-Machine ( STG ), an abstract machine
that makes many of the properties of lazy evaluation explicit directly in the AST.
Code Generation
From the core language we will either evaluate it on top of a high-level interpreter written in Haskell
itself, or into another intermediate language like C or LLVM which can itself be compiled into native
code.
let f x = x + 1
Quite often this process will involve another intermediate representation which abstracts over the process
of assigning and moving values between CPU registers and main memory. LLVM and GHC’s Cmm are
two target languages serving this purpose.
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