Document -pgmL (Use cmd as the literate pre-processor)

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The GHC option -pgmL (Use cmd as the literate pre-processor) is undocumented, and behaves differently than -pgmF (Use cmd as the pre-processor).

While one can count on the first three arguments to a -pgmF command giving file names, with optional arguments coming later, the last three arguments to a -pgmL command give the file names. An undocumented "-h" argument always comes before these file arguments, throwing off any command designed according to the documentation for -pgmF. Moreover, an optional argument provided using -opL comes before any of these arguments.

The undocumented "-h" reminds me of the passage in Real World Haskell, how only one reasonable program could have type "(a, b) -> a", because there isn't enough information to do anything clever. Here, GHC wants the -pgmL command to emit standard Haskell, but for all it knows, the literate program is in Swahili. GHC isn't really in a good position to be offering advice via options on this conversion, because it has no idea what conversion is taking place. So the "-h" is inexplicable.

Also, the example command in GHC manual section 5.10.4. doesn't use argument $1, and probably wants to use $1 rather than $2 for the echo, if in fact it is possible to create circumstance where $1 and $2 differ.

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Example pre-processor that conforms to observed ghc behavior

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Literate version of example pre-processor, requires dedent.hs once to bootstrap

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Test pre-processor to examine arguments passed by ghc