Differences between QuasiQuotes and TemplateHaskell
Hi, Originally, I thought that QuasiQuotes build on top of TemplateHaskell, since they just provide a way of parsing a string into a Template Haskell AST. That is, a quasi-quoter is defined by four parsers of types String -> Q Exp String -> Q Pat String -> Q Type String -> Q Dec that parse EDSLs into corresponding Haskell (abstract) syntax. However, while playing around with the Shakespeare package's quasiquoter hamlet, I noticed that (e.g.) a snippet html = [hamlet| <html> <body> <h1>Hello World |] () has type Html and not (as I would expect) Q Exp. Hence, it seems as if quasiquotes are spliced in implicitly? This confuses me since in Template Haskell in general a quote (e.g.,) [e| \x -> x |] has type Q Exp, which then needs to be explicitly spliced in order to produce the id function \x -> x. So why do quasiquotes not have types Q Exp, but rather concrete types? If anyone could clarify my confusion, this would be great! Dominik.
Counterintuitively, quasiquotes are like TH *splices* not TH *quotes*. This is true despite the fact that the quasiquote syntax looks just like a quote. And they're quasi*quotes*. But they really should be quasi*splices*. Specifically,
[blah|...|]
is the same as
$(selector blah "...")
where `selector` is the appropriate record selector based on the context where the quasiquote appears.
You are not the first to fall into this trap! Though, I believe this is well documented in the manual.
Richard
On Apr 11, 2016, at 9:59 AM, Dominik Bollmann
Hi,
Originally, I thought that QuasiQuotes build on top of TemplateHaskell, since they just provide a way of parsing a string into a Template Haskell AST. That is, a quasi-quoter is defined by four parsers of types
String -> Q Exp String -> Q Pat String -> Q Type String -> Q Dec
that parse EDSLs into corresponding Haskell (abstract) syntax.
However, while playing around with the Shakespeare package's quasiquoter hamlet, I noticed that (e.g.) a snippet
html = [hamlet| <html> <body> <h1>Hello World |] ()
has type Html and not (as I would expect) Q Exp. Hence, it seems as if quasiquotes are spliced in implicitly?
This confuses me since in Template Haskell in general a quote (e.g.,) [e| \x -> x |] has type Q Exp, which then needs to be explicitly spliced in order to produce the id function \x -> x.
So why do quasiquotes not have types Q Exp, but rather concrete types?
If anyone could clarify my confusion, this would be great!
Dominik.
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participants (2)
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Dominik Bollmann -
Richard Eisenberg