Write You a Haskell Stephen Diehl

Expr : let VAR ’=’ Expr in Expr { App (Lam $2 $6) $4 }
| ’\\’ VAR ’->’ Expr { Lam $2 $4 }
| Form { $1 }
Form : Form ’+’ Form { Op Add $1 $3 }
| Form ’-’ Form { Op Sub $1 $3 }
| Form ’*’ Form { Op Mul $1 $3 }
| Fact { $1 }
Fact : Fact Atom { App $1 $2 }
| Atom { $1 }
Atom : ’(’ Expr ’)’ { $2 }
| NUM { Lit (LInt $1) }
| VAR { Var $1 }
| true { Lit (LBool True) }
| false { Lit (LBool True) }
Syntax Errors
Parsec’s default error reporting leaves a bit to be desired, but does in fact contain most of the information
needed to deliver better messages packed inside the ParseError structure.
showSyntaxError :: L.Text -> ParseError -> String
showSyntaxError s err = L.unpack $ L.unlines [
” ”,
” ” lineContents,
” ” ((L.replicate col ” ”) ”^”),
(L.pack $ show err)
]
where
lineContents = (L.lines s) !! line
pos = errorPos err
line = sourceLine pos - 1
col = fromIntegral $ sourceColumn pos - 1
Now when we enter an invalid expression the error reporting will point us directly to the adjacent lexeme
that caused the problem as is common in many languages.
> \x -> x +
\x -> x +
^
”” (line 1, column 11):
unexpected end of input
expecting ”(”, character, literal string, ”[”, integer, ”if” or identifier
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