[Haskell-cafe] Template Haskell: let statement in a splice put in the "main = do" part of a program?

24 Aug 2013

      Hi,

I continue to test Template Haskell, and I have some difficulties to use a 
splice $() in a "do" contained in the "main" part of a program. Here is an 
example. I want to make a splice that does `let a="a"` in my code.

$ cat MakeLetStatement.hs
----------------------------
{-# LANGUAGE TemplateHaskell #-}

module MakeLetStatement where

import Language.Haskell.TH

makeLetStatement :: String -> ExpQ
makeLetStatement s = return $ DoE $ [ LetS $ [ ValD (VarP $ mkName s)
                (NormalB $ LitE $ StringL s) [] ]]
----------------------------

$ cat test_MakeLetStatement.hs
----------------------------
{-# LANGUAGE TemplateHaskell #-}

import MakeLetStatement

main = do

$(makeLetStatement "a")
-- print a
----------------------------

Note I have commented "print a" because otherwise I obtain "Not in scope: 
`a'" that shows that `a` has not been defined correctly, but does not show 
whether my splice has been correctly expanded (I use --dump-splices GHC 
option, but it seems it is not working for splices in the "main = do" part).

I obtain:
$ runghc -ddump-splices test_MakeLetStatement.hs 

test_MakeLetStatement.hs:7:3:
    Illegal last statement of a 'do' block:
      let a = "a"
    (It should be an expression.)
    When splicing a TH expression: do let a = "a"
    In a stmt of a 'do' block: $(makeLetStatement "a")
    In the expression: do { $(makeLetStatement "a") }
    In an equation for `main': main = do { $(makeLetStatement "a") }

That shows that my splice has been correctly expanded: we have `let a = 
"a"`. However, what happens is the same as in the following dummy script, we 
have in fact defined a "do" inside the first "do" (with DoE), and so we 
obtain an error because the last statement in a do block should be an 
expression.
----------------------------
main = do
do let a = "a"
print a
----------------------------

So my code does not work, without surprise, but in fact my problem is to 
transform a LetS statement:

http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell-2.8.0...

that has type Stmt, in an ExpQ that seems to be the only thing that we can 
put in a splice. I have found that it can only be done by doE (or DoE) and 
compE (or CompE) according to 

http://www.haskell.org/ghc/docs/latest/html/libraries/template-haskell-2.8.0...

But doE is not a solution as we have seen above, and compE is to construct 
list comprehensions, which is a different thing.

So, is there any solution to my problem?

Thanks in advance,

TP