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th-to-exp's Introduction

th-to-exp Build Status

th-to-exp is a package that provides a way to persist data from compile-time to runtime by producing Template Haskell expressions that evaluate to particular values. For example, if you have a value Just 1, then toExp (Just 1) will produce the expression [e| Just 1 |], which can be used in a splice. For a more direct example, here’s what that looks like without the quasiquote notation:

> toExp (Just 1)
AppE (ConE GHC.Base.Just) (LitE (IntegerL 1))

This is done by using a typeclass, ToExp, that can be automatically derived for types that have a Generic instance.

th-to-exp's People

Contributors

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Stargazers

avatar Milán Nagy avatar Masashi Fujita avatar YAMAMOTO Yuji avatar

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th-to-exp's Issues

This is Language.Haskell.TH.Syntax.Lift

I'm not sure if you were aware of this or not, but the ToExp class is practically identical to the Lift class from Language.Haskell.TH.Syntax, which can be derived with the DeriveLift extension. (It even has a default implementation.)

Build failiure w/ GHC 8.2.1: No instance for ToExp DerivClause

As seen on the Stackage build server:

[1 of 1] Compiling Language.Haskell.TH.ToExp ( library/Language/Haskell/TH/ToExp.hs, dist/build/Language/Haskell/TH/ToExp.o )

library/Language/Haskell/TH/ToExp.hs:90:10: error:
    • No instance for (ToExp DerivClause)
        arising from a use of ‘Language.Haskell.TH.ToExp.$dmtoExp’
    • In the expression: Language.Haskell.TH.ToExp.$dmtoExp @Dec
      In an equation for ‘toExp’:
          toExp = Language.Haskell.TH.ToExp.$dmtoExp @Dec
      In the instance declaration for ‘ToExp Dec’
   |
90 | instance ToExp TH.Dec
   |          ^^^^^^^^^^^^

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