Re: [Haskell-cafe] Is this a GHC parser error? A test case compiles from one line but not another.

Thanks, Alexis! Your explanation of the distinction between let and where is the first I've understood. On Fri, Sep 8, 2017 at 9:05 PM, Alexis King wrote:

No, it isn’t a GHC parser error.

Your preceding expression ends in a where clause, like this:

assertBool "4" $ p == Right a where p = ...

That’s fine, but make sure you understand how where works. Where clauses are not attached to individual expressions, they are *always* attached to definitions. In this case, the where clause is not attached to the assertion but to your definition of tHash. GHC parses your program the same way as this one:

tHash = TestCase $ do ... assertBool "4" $ p == Right a where p = ...

Therefore, the where clause forces the termination of the do block, since it exists syntactically outside the “body” of a definition. It is its own syntactic construct, an optional sequence of definitions in scope within the RHS of a definition. You cannot have any more expression after the where clause because they would be free-floating, outside of any particular definition.

If you really want definitions that are scoped to that single assertion, use a let expression instead of a where clause:

let p = ... a = ... in assertBool "4" $ p == Right a

That works, since let...in blocks serve as completely ordinary expressions, and they are permitted anywhere an expression is permitted (unlike where clauses, which must “belong” to a top-level or let-bound definition).

...

On Sep 8, 2017, at 20:54, Jeffrey Brown wrote:

The test suite below[1] does what it's supposed to. The last line of it is a commented-out test case:

-- assertBool "broken" $ True

If I uncomment that, it fails to compile:

> :reload [13 of 13] Compiling TParse ( test/TParse.hs, interpreted )

test/TParse.hs:67:3: error: parse error on input ‘assertBool’ | 67 | assertBool "broken" $ True | ^^^^^^^^^^ Failed, 12 modules loaded. >

However, if I move it to somewhere earlier in the code, it works.

[1] https://github.com/JeffreyBenjaminBrown/digraphs-with-text/blob/ e5afd39b950752ccb65997b89ab625d859299b4a/test/TParse.hs

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