Haskell - Happy parser error

Question

I'm working on a project that uses the Happy parser generator. This is what have I done until now:

Exp   : Exp1                    { $1 }

Exp1  : Exp1 '+' Term           { \p -> $1 p + $3 p }
      | Exp1 '-' Term           { \p -> $1 p - $3 p }
      | Term                    { $1 }

Term  : Term '*' Factor         { \p -> $1 p * $3 p }
      | Term '/' Factor         { \p -> $1 p / $3 p }
      | sqrt Factor             { \p -> sqrt $2 p }
      | Factor                  { $1 }

Factor                    
      : double                  { \p -> $1 }
      | '(' Exp ')'             { $2 }

The problem is that I get the following error:

Parser.hs:158:38:
No instance for (Floating ([a0] -> Double))
  arising from a use of `happyReduction_7'
Possible fix:
  add an instance declaration for (Floating ([a0] -> Double))
In the second argument of `happySpecReduce_2', namely
  `happyReduction_7'
In the expression: happySpecReduce_2 6 happyReduction_7
In an equation for `happyReduce_7':
    happyReduce_7 = happySpecReduce_2 6 happyReduction_7

Do you know how can I solve this?

Update: I solved it but now it works only if I write "sqrt2"(no space between sqrt and 2); if I write "sqrt 2" I get "parse error".

This is what I have in the Alex(lex) file:

       tokens :-

       $white+         ;
       "--".*          ;
       "sqrt"                   { \s -> TokenSqrt}
       "sin"                    { \s -> TokenSin}
       "log"                    { \s -> TokenLog}
       @doubleNumber            { \s -> TokenDouble (read s) }
       @var                     { \s -> TokenVar s }
       "+"                      { \s -> TokenPlus }
       "-"                      { \s -> TokenMinus }
       "*"                      { \s -> TokenMul }
       "/"                      { \s -> TokenDiv }
       "("                      { \s -> TokenOB }
       ")"                      { \s -> TokenCB }
       "="                      { \s -> TokenEq }

Answer 1

sqrt $2 p

This calls sqrt with the function $2 as its argument and then applies the resulting function to the argument p. This would only makes sense if sqrt could take a function and produce a function as a result, which would be the case if and only if there was a Floating instance for functions, which there is not. Thus the error message.

What you doubtlessly intended to do was to apply the function $2 to the argument p and then apply sqrt to the result, for which you'd write:

sqrt ($2 p)