Write You a Haskell Stephen Diehl

]
TokenSym ”x”,
TokenAdd,
TokenNum 1
We can then scan the token stream via and dispatch on predefined patterns of tokens called productions
and recursively builds up the syntax datatype for the abstract syntax tree (AST) by traversal of the input
stream.
type Name = String
data Expr
= Var Name
| Lit Lit
| Op PrimOp [Expr]
| Let Name [Name] Expr
data Lit
= LitInt Int
data PrimOp
= Add
So for example the following string is parsed into the resulting Expr value.
let f x = x + 1
Let ”f” [”x”] (Op Add [Var ”x”, Lit (LitInt 1)])
Desugaring
Desugaring is the process by which the frontend AST is transformed into a simpler form of itself by
reducing the number of complex structures by expressing them in terms of a fixed set of simpler constructs.
Haskell’s frontend is very large and many constructs are simplified down. For example where clauses
and operator sections are the most common examples. Where clauses are effectively syntactic sugar for
let bindings and operator sections are desugared into lambdas with the left or right hand side argument
assigned to a fresh variable.
Type Inference
Type inference is the process by which the untyped syntax is endowed with type information by a process
known as type reconstruction or type inference. e inference process may take into account explicit type
annotations.
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