Write You a Haskell Stephen Diehl

e expression or Expr type is the core AST type that we will deal with and transform most frequently.
is is effectively a simple untyped lambda calculus with let statements, pattern matching, literals, type
annotations, if/these/else statements and do-notation.
type Constr = Name
data Expr
= EApp Expr Expr -- ^ a b
| EVar Name -- ^ x
| ELam Name Expr -- ^ \\x . y
| ELit Literal -- ^ 1, ’a’
| ELet Name Expr Expr -- ^ let x = y in x
| EIf Expr Expr Expr -- ^ if x then tr else fl
| ECase Expr [Match] -- ^ case x of { p -> e; ... }
| EAnn Expr Type -- ^ ( x : Int )
| EDo [Stmt] -- ^ do { ... }
| EFail -- ^ pattern match fail
deriving (Eq, Show)
Inside of case statements will be a distinct pattern matching syntax, this is used both at the toplevel
function declarations and inside of case statements.
data Match = Match
{ _matchPat :: [Pattern]
, _matchBody :: Expr
, _matchGuard :: [Guard]
} deriving (Eq, Show)
data Pattern
= PVar Name -- ^ x
| PCon Constr [Pattern] -- ^ C x y
| PLit Literal -- ^ 3
| PWild -- ^ _
deriving (Eq, Show)
e do-notation syntax is written in terms of two constructions, one for monadic binds and the other
for monadic statements.
data Stmt
= Generator Pattern Expr -- ^ pat