I am trying to get a grammar where keywords are also valid identifiers. Been messing round with the following Happy grammar :- %token 'let' { TokenIdent "let" } 'in' { TokenIdent "in" } ident { TokenIdent $$ } int { TokenInt $$ } '=' { TokenEq } '+' { TokenPlus } '-' { TokenMinus } '*' { TokenTimes } '/' { TokenDiv } '(' { TokenOB } ')' { TokenCB } %% Exp : 'let' Var '=' Exp 'in' Exp { Let $2 $4 $6 } | Exp1 { Exp1 $1 } Exp1 : Exp1 '+' Term { Plus $1 $3 } | Exp1 '-' Term { Minus $1 $3 } | Term { Term $1 } Term : Term '*' Factor { Times $1 $3 } | Term '/' Factor { Div $1 $3 } | Factor { Factor $1 } Factor :: { Factor } : int { Int $1 } | ident { Var $1 } | '(' Exp ')' { Brack $2 } Var :: { Factor } : ident { Var $1 } | 'let' { Var "let" } Here 'Var' should be able to represent a 'let' identifier, as well as a keyword. Happy accepts the grammar but it does not parser the expected 'let let = x in let'. I have attached the full Happy grammar. I don't think this is an LR thing, but could be wrong. Many thanks in advance, Aaron