Simple string interpolation

You can find the proper documentation for this library as part of the marvin docs on readthedocs, the following readme provides a shortened overview.

This string interpolation library originates from the Marvin project where, in an attempt to make it easy for the user to write text with some generated data in it, I developed this string interpolation library. The design is very similar to the string interpolation in Scala and CoffeeScript, in that the hard work happens at compile time (no parsing overhead at runtime) and any valid Haskell expression can be interpolated.

The library uses the builtin Haskell compiler extension in the form of QuasiQuoters (QuasiQuotes language extension) and/or splices (Template Haskell language extension)

{-# LANGUAGE QuasiQuotes #-}

import Marvin.Interpolate

myStr = [iq|some string %{show $ map succ [1,2,3]} and data |]
-- "some string [2,3,4] and data"

or alternatively as splice (which might be kinder to your code highlighting)

{-# LANGUAGE TemplateHaskell #-}

import Marvin.Interpolate

myStr = $(is "some string %{show $ map succ [1,2,3]} and data")
-- "some string [2,3,4] and data"

It basically transforms the interpolated string [iq|interpolated string|], or in splices $(is "interpolated string") into a concatenation of all string bits and the expressions in %{}. Therefore it is not limited to String alone, rather it produces a literal at compile time, which can either be interpreted as String or, using the OverloadedStrings extension, as Text or ByteString or any other string type.

i (for interpolate quoter) and is (for interpolate splice) is the basic interpolator, which inserts the expressions verbatim. Hence when using iq or is all expressions must return the desired string type.

There are specialized interpolators, which also perform automatic conversion of non-string types into the desired string type. These specialized interpolators each have an associated typeclass, which converts string types (String, Text and lazy Text) to the target type, but leaves the contents unchanged and calls show on all other types before converting. This last instance, which is based on Show, can be overlapped by specifying a custom instance for your type, allowing the user to define the conversion.

The naming scheme of the interpolators in general is i<splice|quoter><specialization?>. I. e. isS expands to interpolate splice to String and iqL to interpolate quoter to Lazy Text.

• iqS and isS in Marvin.Interpolate.String converts to String via the ShowS typeclass
• iqT and isT in Marvin.Interpolate.Text converts to Text via the ShowT typeclass
• iqL and isL in Marvin.Interpolate.Text.Lazy converts to lazy Text via the ShowLT typeclass

To import all interpolators, import Marvin.Interpolate.All.

Syntax

Interpolation uses the quasi quoter sytax, which starts with [interpolator_name| and ends with |] or splice syntax $(interpolator "interpolated string"). Anything in between is interpreted by the library.

The format string in between uses the syntax %{expression}. Any valid Haskell expression can be used inside the braces. And all names which are in scope can be used, like so.

let x = 5 in [iS|x equals %{x}|] -- > "x equals 5"

There are four escape sequences to allow literal %{ and |]

Input	Output
~]	]
~%	%
~}	}
~~	~

As a result the sequence ~%{ will show up as a literal %{ in the output and |~] results in a literal |]. Note that these are simple substitutions. In general the characters themselves, if not escaped, will not throw errors, aka ~ will be ~ again in the output. Escaping is only necessary in cases where these cahracters would have a special meaning otherwise.

Differences to/Advantages over other libraries

There are a few advantages this libary has over other string formatting options.

1. The hard work happens at compile time

   Unlike libraries like text-format and the Text.Printf module parsing the format string, producing the string fragments and interleaving data and strings happens all at compile time. At runtime a single fusable string concatenation expression is produced.
   Furthermore all errors, like missing identifiers happen at compile time, not at runtime.

2. Type Polymorphism

   The created, interpolated string has no type. It can be interpreted as any string type, so long as there is an IsString instance and the expressions inside return the appropriate type.

   This is different format string libraries like text-format and the Text.Printf module which always produce strings of a particular type, and interpolation libraries like interpolate and interpol which require instances of Show.

3. Simple API and full Haskell support

   The interpolated expressions are just plain Haskell expressions, no extra syntax, beyond the interpolation braces %{}. Also all Haskell expressions, including infix expressions, are fully supported.

   This is different from Interpolation which introduces additional syntax and does not fully support infix expressions.